Congrats on the new iMac :) The zoom features built into browsers don't work with live chess as it messes up the proportions, etc. Go into your live chess settings, and make sure that you have the board set to maximum.
Using Safari on my own iMac with the board set to maximum, this will give you an idea of the biggest proportions you'll see.
Hope that helps!
Got a new imac for Christmas. Using Safari, what is BEST way to get biggest board and smaller other windows? The "green dots" window seems to always be the widest and way bigger than ever needed or wanted. When I enlarge or zoom or command plus, the screens just seem to go haywire. Any advanced Mac users out there can give me some advice? please, I'm new to Macs. thanks so much.