Yes, but it's pretty flexible:
Your roster size cannot fall below 25 players (23 skaters, 2 goalies) after a trade is completed. The number of goalies on your team cannot fall below 2 players after a trade is completed. The number of players (skaters & goalies combined) cannot exceed 35 players after a trade is completed. You must have at least 10 forwards, 5 defenders, and 2 goalies on your roster. Minor league prospects DO NOT count toward these requirements -- they must be real players. This requirement may be not apply if you are in a theme league, as the founder has the ability to turn it off.
And don't worry, you'll be informed of these restrictions if your trade proposal violates any of the requirements.