The "ideal" size depends on the coach...

In reality the "ideal" size is probably about 25-26 players. That way you have enough for full-side scrimmages, and 3-4 extras in case of injuries during the season, poor grades, or those who quit for whatever reason.

In reality you will see many teams north of 30, despite their competitiveness or conference. Many of these kids (male or female) enjoy the idea of being on the team, even though they will see precious few minutes of playing time, in many cases. Or because they believe the coach when they tell them they are their #1 choice (along with the other 10 players they signed that year).

The irony is, that in many of the teams with 30 or so players, there is huge turnover due to the disillusionment, which means they have to have larger rosters to account for this churn, which leads to more churn, and so on.