Is it specifically Sonic's webmail interface that's appearing small or does everything look smaller on the new monitor?
If it's just Sonic's webmail, then you should just be able to use your browser's "Zoom" tool to make things bigger. However, if it's just everything overall is appearing small, then it may be a matter of adjusting the display settings on your Mac.
I found this article online, that has an easy to follow guide with screenshots, if you do need to make overall changes :https://www.laptopmag.com/articles/incr ... t-size-mac
I hope this helps!