Hi Joe...

I have used a 13" for timber stock as large as 150mmx200mm (~6"x8")...So a larger 15" planer would more than handle these in softwood timbers, and with a bit more challenge with heavier hardwoods. Sounds like you are just cleaning them up, and not actually "Jointing" them into perfectly square stock...Which hand held traditional manual planes and/or power hand planners would do just fine for such cleanup work...

If wanting to square timbers that is the job of "a jointer"...and...much harder to achieve with things larger than 150mm x 150mm...

There is (in Japan) a smaller "handle held" squaring jointer/planer for "Neda" "Nuki" and other such members in Japanese framing that is a two sided planer that forms a perfectly fixed 90 degree angle...It still would be a challenge over proper layout methods comparatively...