Rather than set a particular length, do some more homework on what fits your circumstances:
Set a budget
. An older cat will have greater financial obligations after the sale
. A newer cat will cost more in a given size. Leave money
for slip fees
costs, and the inevitables: life jackets, curtains, electronics
, and endless et ceterae.
Where can you keep it? The back yard is out. If there aren't any cat-sized slips near enough and cheap
enough, consider the time wasted commuting. A mooring
implies more complications, such as adding enough solar panels
to keep an anchor
light lit and the batteries charged with enough left over to keep the bilge pump
Buy a boat big enough to accommodate your permanent crew. The number of heads required can be determined by the formula (2F+M)/4 where F= the number of Females staying over night and M is the number of males old enough to stand upright at either transom.
Go smaller, go sooner; nobody buys their last boat first, unless its a terrible mistake.