It really depends on the owners in the league. I've played in dynasty leagues ranging from 14 man rosters to 35 man rosters. All work well depending on what you want. The fewer players on the rosters, the more talent that will be available on the waiver wire. Small roster size also forces hard decisions at cut time. The more players on the roster, the longer you can hold onto those prospects waiting for them to develop, but your waiver wire will be pretty bare. Myself, I prefer 20 to 25 player rosters as this kind of gives you a happy medium from both ends. But, I'd suggest talking with the rest of your league and see what everyone prefers.
Something else you might want to consider is whether you want teams to make cuts this year or not. If you want teams to make some cuts this year to make room for rookies, then keep roster size the same. If you aren't concerned with cutting player's this season, raise the limit a little. You might also consider to a complete re-draft this season, then continuing on with the dynasty thereafter. Many people use different draft strategies for dynasty leagues than they'd use with re-drafts or even some keeper leagues. But if everyone in your league is agreeable to keeping their current roster and going dynasty from there, then there's nothing wrong with that either.
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