Necro thread, but this is what came up as I search for the exact same issue I'm having here today Novermber of 2022, running MiRC 7.71 on a 4k display laptop. Pretty small icons, looks almost identical to your image.
I've been dealing with these super small icons for farrrrrrrr too long. I also had found this setting and it doesn't help whatsover. It might be 1px bigger.
Now we are in 2022.... how is this not a major issue at this juncture and "fixed" for Windows 10/11 etc?!?!?