Cage Too big? and Phasing?

Not sure what you mean by phasing them in. I would think if you want them to live in the same cage, doing a proper introduction would be the wisest move. Then if they can't get along, you can put them in separate cages, with the cages near each other so he gets interaction. But allowing them to go back and forth cage to cage is going to cause huge issues if they haven't been introduced properly.

Putting them in a separated cage side by side is not a good introduction, and when you finally remove the separating bars they will intrude on the others territory, creating more drama. This technique could also cause bar biting which could lead to broken teeth.

A separated cage would be plan B if they still don't get along in a bigger space. But fighting is expected, and never separate in there's no blood drawn. It might look ugly but a bit of furr flying around is nothing to worry about.

A proper introduction could either be when you first put them in their new cage, or similar large new territory. Don't forget to have two of everything such as cozies, food & waterbottles on either side of the cage, and everything will have to be clean & free of anyones scent.

A cage can never be "too large", bigger is always better. ptimist: My 2x5 feels like the absolute minimum for my two girls, and when I'm getting a larger apartment I will absolutely expand to a 3x6. Heck I would give them a whole living-room if I could.

They are brothers and they were once in the same cage, but b/c Thunder wasnt getting much food and was getting thinner while his twin Lightning was getting fatter i seperated them so that i could make sure that Thunder was eating. So i eventually want them to go back to living with each. Lightning has always chewed the bars even when they were in the same cage and i do give him stuff to chew but sometimes he just wants to chew the bars.

You are still going to have to introduce them on neutral territory again. Treat it as if you would when you are introducing 2 new pigs for the first time.

I've never really done that, so idk how. do i just take them out on the floor and let them get to know each other and how many times should i do this?

With guinea pig introductions, less is more. Ideally you should only need to introduce them once, MAYBE twice if there is a fight that draws blood. Introducing them several times will breed hostility between the pigs.
Give this page a read: Guinea Pigs Social Life

Another one that's a must read.
https://www.guinealynx.info/forums/viewtopic.php?t=46468