nah, you are given the size of the UFO in the initial pop-up. in that way, it would make sense to have some kind of visual representation on the globe.
but considering that we have sizes "very small", "small", "medium", "large" and "very large", we would need five sizes of blips - even if we start at 1x1 for small scouts, we end up at 9x9 for the very large UFOs (icons probably should have a "center", so they need to be odd numbers) - that's a really large blip that will potentially obscure other stuff.
on the other hand, battleships are rather rare, and they are terrifyingly large, so seeing a 9x9 pixel blip on the globe gives you the right idea from the start