Remember the physics!!
In a slimline and comfort, you are using a "cross-style" refill. It fits nicely in a 7mm (.2756") tube. LOOK at that refill, it is less than 1/8" diameter.
Now look at the international cartridge or a converter. Just under .3". So, the tube needs to have an internal diameter of .35" or better. The finished outside diameter of the pen has to be .45" or better (including brass tube).
Yes, those who have used rollerballs and fountain pens are, usually comfortable with this size. They MAY complain about the weight though, because commercial pens do not use brass tubes or weighted tops, so the commercial pens ARE lighter.
A well-balanced hand-made pen will be comfortable--just get your prospective customer to write with it.
Yes, they CAN be sold.