Fleet size can be highly useful against TEC and Vasari players. However, you can't throw just sheer numbers at an Advent opponent and expect to win. If I'm playing Advent, the bigger your fleet means the more ship kills I get due to all that damage propagation and retaliation Advent capital ships have.
To answer your question though, maximum fleet supply and maximum capital ship slots are applicable to all size maps. You can have the fleet cap on an 8 planet system or a set of stars with 152 planets. The trick is being able to afford having the higher level fleet supplies. On larger maps it's a bit easier since you have more planets and more income. On smaller maps you'll be squeezing your income so much that having a really high fleet supply will cripple you instead of help you.
