There should probably be an upper limit on troops, because they start slowing down the game a lot rather quickly (and spamming them after maxing out the upgrades seems to be the way to go for winning). It also freezes for some reason when trying to exit from the pause menu.
Fun concept, though. Wasn't expecting the RTS mechanics when finding this in the Tower Defense tag and they worked well. I don't think the map needs to be bigger than what's on the screen - I actually didn't know it was bigger until I read the description after completing the game (the rest is very intuitive, evidently!).
EDIT: Alternatively, maybe the troop cost could scale depending on how many there are, just like the upgrades get more expensive. I ended up with a lot of "dosh" in my playthrough, to the point where it became irrelevant and I could just spam-click the troop button.