A high theme square count can be an asset, but there are pitfalls galore:
1) All the entries must be winners. If to get up to 5 or more entries in a 15x15 you have to pad your theme entry set with some entry or entries that don't quite measure up or are inconsistent with the rest of your theme entries, you're doing yourself a disservice. (Of course deciding which entries are winners is a very subjective business. I don't mean to oversimplify here.)
2) Your theme entries must be symmetrical. There are occasional exceptions to this rule, but if you think you have one, it would be a good idea to get some advice from a seasoned constructor before submitting.
3) Something I learned early on is that editors don't welcome short extras, especially if the extras are not symmetrical. There are two exceptions to this rule of thumb. First, a short central "bonus" entry can be a plus. This, at least preserves symmetry. Second, if your puzzle is running without a title and the solver is going to need some help in finding your theme, a short helper entry is a must. Occasionally a helper is even necessary with titled puzzles. Most editors like the short helper to be in the SE, if possible. The reason is that this location gives most solvers a chance to unravel the theme on their own before getting to your helper. (The worst place for a helper would be in the NW.)
It's tempting for rookies to think that they can add lots of short extra theme entries to a grid. In fact, locking yourself into short entries makes filling a grid MUCH MORE DIFFICULT.
It's also tempting for beginners to think that the more entries that are at least obliquely related to their themes, the better. This is not something that editors look for in most themed puzzles. It can be downright disadvantageous if the theme requires knowledge of something like books, movies, operas, or what have you. You don't want to stack the odds against someone who is not all that familiar with the topic of your theme.
4) If your theme square count is so high that your fill suffers then, in general, you've done yourself a disservice. There are exceptions to this rule too, of course, but by and large the quality of your fill is more important than a high theme square count.
Some good theme entry lengths for the beginner are:
3 entry sets:
Central 9's are quite difficult and central 11's are even worse, so it's a good idea to stay away from these at first.
Note that your central entry must have an odd number of letters for reasons of puzzle symmetry.
4 entry sets:
Be aware that, with 4-entry sets, if all your theme entries are longer than 11 letters, you lose the ability to place theme entries in rows 3 and 13. Once your outer pair go in rows 4 and 12, your theme squares have less separation between them and the resulting fill is much tougher.
Another rookie mistake that I made a lot when I was starting out is the temptation to try and have theme entries cross one another at every opportunity. This makes filling a grid much tougher, so beware.
A final warning for themed puzzles--your chances of selling a 78-worder with a sparkling fill is much better, in general, than selling a puzzle with a lower word count but with some iffy fill entries. I think it's especially important for novices to take this to heart. You won't impress an editor with what I call a "muscle grid," if the fill is not up to snuff.
Please understand that this is all general advice. There are exceptions to everything I've written.