It's not so much a matter of size as much as logistics. If you can position your speakers so you have as much "sweet spot" as possible, you're in good shape.
You should have the center in front of you, on top oor under the display unit, the mains flanking that, as close to equidistant as possible, and your main rear surrounds above, and preferably slightly behind, the listening position.
I have to scrimp on the surrounds by having them alongside my listening position. No way to put 'em behind at all.
That 6th speaker in a 6.1 setup should be centered somewhat well behind your listening position.