New bottom bracket?

If i was to buy an old road bike to refurbish into a fixed-gear, does the quality of the bottom bracket matter?

Would i possibly need to replace it with a new better quality one?
And if so, how many different sizes do they come in etc

AND does the size of the bottom bracket relate to what size chainrings and cranks you need?

Nah it doesn’t matter that much. If it’s an old roadbike (like 70s or 80s), it will probably be an old cup & cone bracket which can often be rebuilt (it’s not hard). Chances are it will NEED overhaul anyway.

There is some impact on the type of cranks you can use (chainline) but you can usually replace the spindle and/or fiddle with various spacing techniques to get that right.

Do you know what kind of bike it is?

Well does a nice smooth drivetrain matter to you?
The fact you have an old road bike suggests that you probably don’t want to spend too much. What’s the bike for? If it’s just a street-beater and the cranks turn without too much resistance, they’ll probably be fine. Try cleaning the bearings if need be.

The size (spindle length) does relate to the kind of cranks you can use.
The taper will also determine the compatability.
Oh, the threading needs to be correct too!

Type “Sheldon Brown Bottom Bracket” and read for more info.
Tell us more about the bike and the current BB.

Havn’t actually got one yet,
just thinking aloud…
but yea probably just an old 70s/80s road bike.
Nothin too serious, just for beatin around the place.
I could just rip out the old Bracket and replace it with a basic new one yeah?

If you wanted to run a proper track crankset, a new BB might be desirable.
If you wanted to use the existing road cranks, you’d keep the existing BB as well.

didnt wanna spend $600 on a new frame. might upgrade sometime in the future.
I’m thinking i’ll just use the old road-bike’s frame and replace most of everything else.
Bracket, chainline, cranks, wheels etc

As this is all rather hypthetical, it would be best to see what frame you end up with first, and then decide on what components you might need.
For instance the road frame might come with some perfectly useable Shimano 600 cranks and BB that you could put a track chainring onto. There’s half the bike already.

Conversions are ALWAYS a can of worms. But embrace the worms and have fun.