10mm scale is no problem at all. Above 18mm, the issue of proportions becomes a problem with figures optimised for 6mm. The converse is also true. Simply reducing from 28mm to 6mm will not produce a figure that works, both from a printing and aesthetics perspective. Some people have printed at 20mm by reducing the X- and Y-axis dimensions compared to the Z-axis. The latter gives the height of the figure, so the front-to-back and side-to-side dimensions are kept more narrow than they would otherwise be. This doesn't work well for 28mm though, as weapons, etc all appear disproportionately large.
Robert