I'd look for a boat in the 25 to 30 ft range
Depending on how primitive you want to make it, a Catalina 22 could be a option also. The problem with a boat much smaller than a 25 ft'er is that you'll lose a head (bathroom), a true ice box and a cooking surface may not available. Headroom (standing) will be an issue. With no head, you'll have to make do with a portapot. Not bad, but not a lot of privacy either. If this isn't problem for you or the GF, then I'd recommend a Catalina 22.
The Catalina 22 is one of the most successful sailboats ever made. Easy to sail single-hand, comforatble to sail, room below to sleep for two, but no real cooking area or cold box. It's a basically daysailor that can be slept on.
Would need more information on recommended boats for you such as:
Where are you going to sail, do you want trailer your boat, do you need a functioning head (enclosed), cooking area, how often do you want to overnights, etc.
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