Ideal text or font size in Adobe illustrator for export to Rise?

Hello, I am uploading labelled diagrams created in Adobe illustrator into a Rise module. 

Sometimes, these look great with clear, crisp font. Other times the text comes out pixelated and grainy! I experimented with different font sixes in the export from Adobe illustrator, but I can't seem to find an ideal size that always works well. 

 

Attaching screenshots of good and bad examples.

 

Thank you - this is driving me to distraction!