Whilst deciding how to model Traveller starships in my YAGS RPG, the issue of what scales to use for both time and distance came up. Traditionally, the Traveller combat rules used 1000 second turns (a little under 17 minutes) and 10,000km as the base distant unit. Short range for starship weapons is around 100,000km, long range about 300,000km.
This gives a nice grand scale for space combat, and avoids the big complaints about Hollywood SciFi of space ship weapons seemingly having shorter range than modern personal fire arms. However, it has a couple of downsides.
What follows is a look at various possible scales that could be used in Starship combat, from the point of view of an RPG. For these purposes, I'm not so much focused on realism as looking at the story telling consequences.
I do assume a few Traveller 'facts' throughout. Notably, a ship must be 100 diameters from a planet to be able to Jump (assume 1,000,000km). A fast ship can accelerate at 6g. A fast ship can move out of effective range of small spaceship scale weapons in a few turns.
In Full Thrust tradition, I represent a unit of distance as 1“. This is the distance on the tabletop when playing out ship to ship combat using miniatures. It may represent '1 inch', it may represent '1 cm'. Whatever works for you, but it's a shorthand for the smallest in-game distance that can be easily represented. It also defines the largest region of space that can be easily represented on a typical dining room table.
In Traveller, a turn of combat lasts over 15 minutes. This is fine for grand fleet battles, where giant behemoths manouevre slowly to get the best tactical position. However, in most RPG campaigns, PCs aren't commanding huge fleets, but are flying small trade ships or fighters, smuggling goods between worlds or avoiding pirates.
1” represents 10,000km, so an Earth-sized planet is about 1“ across, which isn't much of an obstacle. The distance between the Earth and the Moon is some 400” (10 metres), so way bigger than most people have space for.
Take the stereotypical SF situation - a group of PCs shoot their way past guards at the spaceport, and quickly take off, accelerating away from the planet in order to reach a safe distance to Jump. Whether they can escape depends on how long it takes local defences to mobilise, and how quickly the PCs can run.
With 15 minute turns, the local space port can easily man its defence turrets, launch interceptors and generally call out for help before the PCs have gone anywhere. The PCs are probably dead.
If we assume that the PCs first disabled the space port's defences before attempting their getaway, and blocked off the launch pads for the local police ships, then all they need to worry about is the patrol cruiser in orbit. If they can avoid that, then they're home free.
If we assume the patrol cruiser only has a range of 10“ (100,000km at this scale), then it can hit anything in orbit around the planet. An Earth sized planet is only about 1” across at this scale, the PCs with 6g acceleration can move 3“ from the planet in the first turn, a turn in which the cruiser has ample time to notice what is going on and shoot at them. Even if the PC's try to keep the planet between them and the cruiser, the cruiser only needs a 2g acceleration to be able to get clear and fire a shot.
There is a relatively small shadow into which it can't fire, and with 15+ minute turns it has plenty of time to detect the escaping ship, warm up its weapons and fire. It also doesn't need to move far to target a ship in the planet's shadow. Two such cruisers can easily blockade an entire planet.
Realistically, a decent cruiser will have weapon ranges possibly of 30”+, and get multiple chances to destroy the PCs before they reach the 100“ Jump distance. Again, they don't stand much of a chance unless they have a warship themselves.
What does this mean? Firstly, there is not much opportunity to sneak between patrol cruisers during smuggling runs, and likewise little opportunity to pirate. Except for gas giants, planets have little effect on combat.
There are no hotshot pilots - decisions take 17 minutes to play out, ships cannot see each other, and even fighter combat is going to be between points of light dancing in the dark. In fact, it's hard to fathom how fighters could be useful at this scale.
When playing Full Thrust, I generally assume that 1” = 1000km. This means planets on the wargaming table can be represented as globes 10“-15” across - small enough to fit on the table, large enough to have a noticeable effect. Shorter range weapons have 10“ (10,000km) range, bigger weapons 30” (30,000km).
Using 1g as the base unit of acceleration, gives a turn just over 5 minutes in length (the actual number is not very pretty, about 316.2s).
We still have the issue that 5 minutes is a long time, and probably long enough for people to activate defences, though a backwater space port may take a couple of turns before they are operational. Planet side weapons can reach well beyond the atmosphere, maybe 30,000km into space, but only a small fraction of the 1,000,000km to the Jump limit.
As for the traditional Traveller setting, a couple of patrol cruisers are enough to completely blockade a planet. However, the weapon range is now more limited. They can attack anything launching from the surface, but with 6g acceleration a fast ship is 27“ away in three turns, and out of range after that. Still a good chance of getting destroyed, but it has some chance of escaping.
Space distances still feel big at this scale, but five minute turns doesn't feel very action orientated.
The default starship scale for YAGS is 1km range units and 10 second turns. This is radically smaller than both of the above.