Kyle & I have happily shared a 25' mono, a 30' mono and now a 33' Catamaran. You absolutely make it work whatever the space you have. In each boat we have spent months of living aboard and month at the dock. If you plan to be away from the dock you need the extra ability to charge batteries (if you have a motor, that will do it, but you might prefer a solar panel or two). Space! I have lived alone in 5bedroom houses and wished I had more space. Small live aboards with a partner will only work if you are both 'up-for' the life. I lived aboard without refrigeration for 5 years - you can make do (and live happily) with very little. Now I have a refrigerator and especially enjoy every ice cube made! Hanging space is overrated - you wear clothes longer before laundry runs, and you migrate your wardrobe (and/or your standards) so you don't need to iron / hang clothes. ACCESSIBLE storage space is always useful. And always bag/tub EVERYTHING that would be damaged it if got wet/damp.
What space is ideal? The one you can afford both to purchase AND to maintain. If you really need convincing – read “Sensible Cruising: The Thoreau Approach by Don Casey and Lew Hackler”
Dock living is not that expensive if you are not also paying for a house of course you pay by the foot, so less is cheaper. Additionally but each area has different property taxes – so the more inexpensive your boat originally (be it small or larger but older) the less you will need to pay.