ttt as I can't help

Depends on the size of the towel.
Sitting in front of my laptop I am not sure how big I put a design on a bath towel. 10-15 cm probably, it also depends on the design. Flannels I do about 3 cm. I have only put a single alpha on a towel. If it was for a child for school I would do it very big, just so they could see it very easily
Someone else will be far more precise than me

lexus I usually look at things in the store & see what they are selling to get a perspective on what I am going to do. As asterix said, a lot depends on the size of the towel. Towels in store have all sizes. If u can print a template out of what u want to do & lay it on the towel this will give u a good idea of where to start in how large or small. *4U

I make sure the whole design will show when the towel is folded in thirds, as you would do if you were hanging say 2 towels to a bar. So the size of the towel determines the size of the design. Unless of course you are putting a design all across, perhaps in that flat woven, non-loopy part that separates the margin from the main towel, then go all the way across with a small design. Good luck! Hugs, Marji

I look at the density of the design. If it is to loss after a few washings the towel will start to show between the stitches.

I do a lot of children's hooded towels & use the 5x7 hoop..for a design & name together. But never go beyond the 5x7 range on regular or it won't show when folded to hang if it's a towel without a hood.

I too, use as much of the center 1/3 of the towel as I can. My monograms are always 5-6 inches.