Whats the biggest we could build a Solar Sail, and how fast would it go?

Hypothetically a solar sail could by made as big as you need it, but any bigger than a small city would start to have an unreasonably high probability of impact with meteorites.

Solar sails are slow. They take years to get up to speed, and therefore are not a good choice for manned spaceflight. They're also next to useless outside of the orbit of Mars, where the sunlight become too disperse. You could use solar sails for interstellar travel, and they are a pretty good choice for that, as long as you don't need to reach your destination in under a century.

I prefer nuclear pulse propulsion for long range travel. Old bang bang.

How long is a piece of rope?

Consider:

1. Solar winds go up to a million miles an hour.
2. Ice boats (sailboats on skates) can actually go faster than the wind!

How fast do you want to go? How long do you want to wait?

Still take 1000s of years to get to the nearest star

In theory there is no upper limit. It would be punctured by debris but not quickly.
In practice we have no materials that are thin enough AND tough enough AND reflective enough to get any sensible acceleration.
They also only work going out - the inverse square law operates - the energy driving it rapidly decreases once it HAS accelerated.

Insanity. Planet Earth's resources are running out. We need to consider mining in outer space.