I'm thinking of buying this kegerator on Amazon, but was wondering if the smaller tank size will make it harder to find a place to fill it. Also, if I'd like a bigger tank at some point will the regulator and tubes still work?
A CO2 regulator has a standard connector which would fit any size CO2 tank you'd be able to handle at the home level.
Most people start with a standard 5 lb tank. Its about 6 inches wide, and 1.5 feet tall. The next size up is a 20lb tank. Its something like 2 feet tall and 8 inches wide. Those are the most popular.
I strongly recommend finding a place that fills tanks before you buy a tank. Near me they only do exchanges. SO you pay a one time deposit on the tank, and then its $12 to swap said tank for each "refill".
If you do find a place that does refills, they'll have no problem filling any size tank, so I wouldn't worry about that.