Pentel's Oh! Gel retractable gel ink pen is one of the largest pens, diameter-wise, that I've used in a long time It's a comfortable pen, the grip is soft rubber and the pen itself has great "hand feel" to it--not too heavy, not too big, but definitely noticeable next to all those skinny Pilot Hi-Tec gel inks.
Staples carries 5 packs of these pens, one pack in all black and one in a black-red-blue assortment which is what I bought. This blue is very dark, one of the darker blues I've seen in a gel ink pen that isn't a blue black. It's also got a thick, 0.7mm line width that isn't too big for my taste. The ink dries fairly quickly, so smearing shouldn't be a problem. The barrel has a nice diamond patter that's distinctive, while the clip is substantial without being heavy.
If you're looking for a substantial gel ink pen, one that you can't possibly loose among the clutter of your desk or bag, the Pentel Oh! Gel ink pen is something to check out.