Hi Boatman, glad you're liking it, and neat to see another user making maps for their PnP campaign
To your question, REXPaint will export the image at whatever font size you're using at the time, so one option is to increase the font size before exporting, though you may have already tried that and the maximum available character dimensions are technically limited by your display (which might play a part here in case you happen to be using a laptop or smaller device).
One option in that case, if you're not already using the largest available bitmap font, is to technically enable the advanced unlimitedFontSize feature described in the manual, with which you can zoom to levels beyond those supported by your display (generally using the keyboard-based font adjustment option, pgup/dn).
Another more flexible option would be to use an outside solution, taking the exported image and using another paint program (or some free online version thereof) to double its size, for example. (Preferably using pixel-perfect scaling, to preserve the crispness, though not necessary, of course.)