Team size

What size of team should you have? By default, the system gives you 18 players, and you cant go under this figure unless a player retires. Above 18, then you can sack players if needed.

Small is beautiful
The smaller your team is, the more matches your players play, the easier they get their 100% experience and the smaller your total form training costs are. So, try and have a team size of 17 - 18.
You should actually try and get the following set up for your team: develop a team with 1 goalkeeper and a low skilled junior reserve goalie; 5 defenders, 5 midfielders, 3 forwards: plus one allrounder.

Preferably, the allrounder is a midfielder or defender. A 7 skilled allrounder will play as a 5,6 skilled player in any other position. Total: 16.
Add 2 or 3 juniors to your squad.

By the way: some managers take an extra risk and play with one keeper only. Thus, they save 0,47 mil on training cost, per season.

Ofcourse this is my personal opinion however dont try and have too big a squad - keep it as small as possible.