When you first select the large font it's fine until something makes the font window resize itself. It can be an event like moving the the effect setting window, installing a new version of xlights. I haven't checked which models it effects but this mainly seems to happen on a large Matrix or P10 panels. But maybe that's because that's the only time I'm trying to use large font.
When I get home tonight I can check to see if it has the same effect on a smaller model.