How to render specific parts i recently noticed i was rendering in half of 1080p, those renders alone took about 4 hours. with it in full 1080p it will take 25 hours just to render 300 frames! can i render a specific amount of frames at a time so my PC isn't working for 25 hours flat out? thanks.
You can do all of those things! Generally, it's best to render animations to a still image output. That way, you can render some, then come back to it and render more frames. 

As for the resolution, it's entirely up to you. If you personally think that 720p looks good, go for it. If you plan publishing this on YouTube or putting it in a reel, you should render at 1080