This may help you. I have the Makita 1806B (6-3/4" power planer) (and a tiny Bosch 1594). The thing is a brute and is massive. I have seen the Makita KP312 (12-1/4") and really can't imagine using it. Mafell and Hema make even larger ones.
Is the big Makita worth the money? That depends, are you milling a lot of beams on site or how wide are your slabs? If you're not, I'd suggest another route.
If you didn't know, planers do not "flatten" pieces really. They make sides parallel. So, these power/portable planers aren't really planers so much as portable jointers (which do flatten things), but the short decks aren't super great for that either. Normally I'll use a router jig to flatten the slabs and the 1806B to "flatten" and finish the last 1/64" of the slabs I cut (up to 50" wide). Also, it is still reasonably useful as a portable jointer. It is also huge 21" long and 20 lbs. The KP312 is the same length roughly but double the weight... not something I'd like to lug around all day.
Even with those tools, I still have a set of old Stanley hand planes and jointers around to help out. Those, and a Dewalt DW735 so when I get one side flat I can make the other side parallel (it's what a planer does).
Good luck on whatever you choose. Just know what the tool actually does before you buy it. Don't go in thinking these things are going to make a slab dead on level.