When I level im not sure what size plane it can give…is it always small?

There are a couple of cases where a M aircraft is given but generally its S aircraft as they are cheap within the game

Are you talking about the levels where you get a free plane to own, or the ones where you get a special arrival?

You get two free planes at each airport. The early gift is always small. In INN and BRI, the later gift is also small; at every other airport it is medium.

For the special arrivals, they get larger as the levels go up.