Resizing more than roughly 50 % will cause ugly ugly artifacting because of filtering. It's particularly evident in fonts because of the sharp edges and the way we read them, as opposed to regular old textures which can blur much more forgivingly.
In smaller fonts, you don't want to be resizing it at all, since it corrupts so easily.
Figure out how many sizes you need, and make as many as you must - resizing from 60 to 40 is doable, but from 10 to 20 or the other way is bad.
I'm currently trying to get fonts sorted out in my project using the method detailed here, but I'm having some issues.
Originally, I had a single font in my atlas at size 64 (the largest I ever wanted) and then I would set the scale on labels to the desired sizes (e.g. (40,40,1).
I was finding that this was not very crisp on screen when running on anything below my game's native resolution (2048x1536). It wasn't TERRIBLE, but just not as nice as I would like.
I then stumbled on this thread and figured this would be the answer. I created multiple fonts in my atlas - e.g. font35, font45, font55, etc. When I then use a font (e.g. font45) I set it's scale to match (e.g. (45,45,1)).
What I'm finding though is that it's a lot worse than before - much blurrier, less-crisp, etc.
So, I wanted to clarify that there aren't any other "steps" that I'm missing? Something that I could be doing wrong?