How small is too small?

[James Stout]

I once had the slightly miserable experience of renting a studio at less than 100 sq. ft. for $1,000/month.  But that was Hong Kong, and this is San Diego.  Some studio/ 1br+1ba properties have piqued my interest here as of late but the question remains: how small is too small?

For those of you who may have some experience in smaller units, how small is too small with respect to square footage to have a marketable rental?

Clearly location and condition of the property are factors.  Let's assume both are excellent.  What other factors are must-haves - rent them furnished, special amenities, etc?

Thanks!