raspbian or not

I have a raspberry pi 3
I find some of the information I got confusing.
I use raspbian, and have a 16, 64 and 128 gig sd card; which is relevant to my questions.

Ideally I would use a 128 gig card, install raspbian and then install librelec onto that. Owing me a ton of space for apps and ram/video ram. The information I got off the wiki gives instructions how to install via raspbian.

Another option is just install libre elec onto its own card, but if that’s the case I would rather not use my expensive 128 sd card. How much space do you really need, if you max things like the video cache?

Which method is best ? Which size sd card, and how can i designate most of it for video memory when using kodi?