How can I change the sizes of partitons? I want the system partition to be bigger specifically.

[willmon22]

I have seen various ways like the bootloader thing but it doesn't give a big enough size. As my stock size is 350 and the biggest for the bootloader thing is 250. So I am wondering if someone knows how could they give me the steps to do it? Or if you could make a image to flash to change the sizes that would be great. I am just wanting to be able to flash kitkat with the full gapps. No matter what I do I can't seem to get google play to get on there even with the slim apps. Any help would be greatly appreciated thanks.