Regarding ship size/number of pax.
Two types of cruise - expedition and "cruising".
In general:
Expedition - you'll be out on zodiacs landing at beaches/rocks, motoring a shoreline; groups of 10-20 ashore. Has to be small number of pax on board; so more expensive per pax. More remote, hard-to access areas.
Cruising - you'll be visiting glam ports for day trips ashore. Busses or do-it-yourself or nothing. Can be large number of pax on board, so cheaper per pax. More relaxing.
I go expedition - 200-300pax, Ponant and SilverSea so far. Always at least 3 restaurants, a pool, no casino, you can socialise or not. The maître de knows you and greets you by name. As a single, you meet others of like situation and eat together & socialise. But if you don't appear, no-one stresses. I've made some friends aboard so far, no real PITAs-es.
Exceptions:
This Christmas, doing a Caribbean cruise; 600pax, Silver Moon. It'll be OK.
AFF cruise: Crown Princess, 3,000 max
Hey, I didn't organise it, and the only stop is Hobart