Sorry for coming into this thread a bit late. You basically found out something that's fairly well-known (at least for many of the font people) about emoji fonts, or bitmap fonts in general. Most Emoji fonts internally store the glyph shapes as colour bitmaps - png, sbix, etc, so they are fixed resolution. The only known exception AFAIK at the moment are a few emoji fonts from Mozilla/Adobe camp, which stores them as SVG's. As general bitmap fonts go, obviously trying to load them at a size not at one of the designed size, would fail with an "invalid pixel size" error.
Yea, it was stupid of me to try setting the size of a bitmap embedded font, whose glyphs are fixed size. Lesson learned! :)
Also, as far as I understand now, with the case of bitmap embedded fonts, a user can look inside `face->available_sizes' to list all the available sizes and then set the one the user desires. Am I correct?
Regards,
Moazin