Whats’s the best spec for a macmini 2 display? What cable is required? Limits on size of display?

You can connect most any aftermarket external display to the new Mac mini.

see > Connect a display to Mac mini - Apple Support

You can even use an HDMI equipped TV to get setup, while your in the process of shopping for a nice 24" thru 32" Display to go with the Mac mini.

When I purchased my M1 mini, I hooked it up to a 40 inch TV via HDMI and the clarity was poor, and I just knew the eyestrain would be horrible. So I decided upon a 32-in LG 4k HDMI Ultrafine anti-glare display that I connected to the mini with a Belkin UltraHD 4K HDMI 2.1 High Speed cable and the result was amazing. I traded the M1 mini for a recent M2 Mac mini Pro and still use that LG display. LG has several 32-in displays across all price points, and that 32-in workspace is a reward over the 27-inch display.