This can be such a pain in the rump. In my game - Iron Infantry - deciding how big to make the men near the tank - a chore - how big the heli - near the tank - near the man - a chore - it took a while - make the tank move not to fast - not to slow - same with the man - same with the heli. Once I had these figured out - I could kinda come up with MY OWN measuring ... like - Ok - 2 game units is 9 feet... or whatever it is/was.
Then I got all my bullets lined up, and positioned the guy when in the tank, all that jazz - but sound was acting really hosed - so were some camera jiggles I was getting.
Someone said "DUDE - when you have stuff that small - rounding errors and stuff might mess ya up"
So I made things Bigger - tanks now 6 units wide - (Note - some people - makes sense too .. make a game unit equal .. in their mind... a unit they understand - like a meter. then they size things so they look right near a "meter stick" or 3d ruler of some sort) then I had to realign the men, the bullets - because scaling the offsets I was using to line stuff up - was close - but not dead on correct.
Very Good Question and I like how TDK clearly pointed out its about what you see - or what you think you see - that gives it scale.
Note though - my point is that making things .5 wide - like a truck - might not be ideal for an FPS for example - sounds don't seem to position as well with small fractions - there is NO RIGHT answer - no RIGHT SIZE but (not just me) people have better results when things are a bit biger than .5 for say a truck.
One rule of thumb - is a unit a meter works well - but like TDK said - its relative. This 1 unit per meter is probably not a great idea for a space game. Your camera rnage would have to be set... um... yeah - I'm sure you understand
Good Luck!