OK, some thoughts...
A mortar & pestle needs to be always clean, always handy & always the right size for the job.
A teeny mortar & pestle rapidly becomes a decoration rather than a useful tool if it's not big enough to cope with the high volume tasks. On the other hand if all you're grinding is 4 cloves, then you'd be lost in a large one... so you may end up with two.
There's always a lot of 'elbow grease' involved to use it & to clean it.
If you're happy with that, read on...
Personally, I went with a resin one. Looks like stone, feels almost like stone, actually man-made.
From comments - I had an untested theory that a real stone one could potentially be tainted by strongly flavoured ingredients, so I went with resin, as being completely impermeable.
Weighs a ton, or feels like it. I just weighed mine to see... my kitchen scales wouldn't take it, so it went on the ones in the bathroom - 7.5kg, that's over 16lbs.
I found a pic of one a bit like it...
Heavy, so it doesn't run away from you when you're pounding like a champ.
I used to have a smaller & much lighter one & I found I was chasing it round the worktop more than I was actually usefully grinding. Also, it was wooden, so every time I cleaned it I couldn't use it again until it was properly dry or I'd be making sludge. I eventually binned the smaller one as I almost never used it, & because I have no qualms about using an electric grinder, chopper or blender as appropriate - & in fact I got an electric spice grinder the last time I had just 4 cloves to grind & could barely find them in the bottom of the mortar ;)
The trouble is that even at that weight, it will still really only hold about a litre [2 pints or so] whilst giving you room to manoeuvre.
For similar reasons I also bought an electric blender big enough to do large batches of soup.