Question

I've always done map cinematics by just going letterbox mode, and unit transmissions, I have never really tried using cinematic mode. So whats the difference?

The cinematic mode does a numer of thing, which can all be done by separate actions.

Besides the letterbox mode, it also disables user control, removes fog of war, black borders, changes the volumes a bit (music lower, speech louder), and maybe some other things I forgot.

It's much more convenient to use the cinematic mode.

Ahh, I see. So if I were to make a trigger using seperate actions to do cinematic functions for my maps, would this have an effect on the size of the map and/or map performance?

No. The increase in size, if there is any, is very very insignificant.
The only real difference is that you only have to add one action. There is no need to fix your older maps.