A big thing you need to worry about, is fleet composition. But, having enough fleet is also a really big deal. A good way to figure how much fleet you have/need, is by how much fleet you have per 1 economy. I have a list of how much is good, bad, etc.
FLEET PER ECO = f/e
Take your fleet size and divide it by your economy.... example:
Fleet size = 1,000,000
Economy = 3,000
1,000,000 / 3,000 = 333.33
<150 bad - needs work badly
275 better - still needs work
300 ok - minimum number to strive for
450 average - like it says, average...
600 good - this is where you should try to be
900 great - this will give you a much better balanced fleet
1200 excellent - strong fleet providing offensive and defensive fleets
1500 awesome - not many have a better ratio than this, but they ARE out there
1600+ ideal - This is what we all need to work toward. Once we get here, we will OWN.