how big does my ship haveto be to accept upgrades?

What kind or upgrade? If its modules, just as big as the module requires. If its rank points, theres no limits.

You ship should start with enough space to fit some basic modules at the beginning of the game.

For instance, you start with the ability to build autocannons, autopilot consoles, titanium plating, proximity scanners, and proximity jammers. These are small enough such that you should be able to build a few of them right away.

If you have run out of space, you have a few options:
* You can 'uninstall' one or more of your existing modules. This puts them in your cargo for use later.
* You can sell some of your modules.
* You can gain additional ranks and increase the amount of deck space available.
* You can acquire some artifacts that increase the size of your ship

Do ships get bigger? I'm stuck on a scout ship. And if ships do get bigger is there a benifit for it?

you have to build decks to make the ship bigger. the benefit is you can put more and bigger modules on your ship, such as energy, scanners, weapons and more. a bigger ship is more versatile and more powerful in many ways...

I forgot what it was for light frigate but I know 100 is frigate.

I think it's every 50 levels, i.e.

1 - Scout
50 - Light Frigate
100 - Frigate
150 - Heavy Frigate
200 - Light Cruiser

etc.

yup