I’ve been ‘collecting’ blocks off eBay over lockdown and so can compare Harken, Lewmar, Holt, Rutgerson and Selden!
For the price, I think the Selden plain bearing blocks were the nicest. Have a look at the ‘PBB80’s. Their fiddle blocks also have a good SWL and aren’t as eyewateringly expensive as some of the others.
I’d go for 70mm+ based on the risk of shock loads with the boom and being able to comfortably accommodate 10mm or 12mm (which some find more comfortable to handle). But I guess a windage and performance argument could be made for smaller. I’m no expert whatsoever, as my attempt to calculate purchases attested! ?