Minimum length for going into the ocean?

The shorter and stubbier the yak the more of a tub it will be.  It's the aspect ratio (length/width) that determines how easy it is to move on the water.  The higher the aspect ratio the easier it is to move.  Think about it as how much water you have to move out of the way.

A Hobie Adventure has an aspect ratio of 192/27.5 = 6.98.
A Revo13 has an aspect ratio of about 161/28.5 = 5.65.
A PA14 is 162/38 = 4.32. 
These differences are actually a lot and you will feel it.

After pedaling a tubby Revo13 for a few years I can't imagine transitioning to a PA14. 

As more folks are joining the forum I am seeing a lot of emphasis on "stability" and I think it is false security.  Even a newb, after having their yak out four or five times will gain a lot of experience and their idea of stability will change.  There is a trade off between stability and effort.  More effort means more fatigue, lower range, and less ability to deal with adverse conditions.

Someone needs to come up with an aspect ratio versus height and weight equivalency.  In a Revo13 a little guy of 150 pounds with 50 pounds of gear is a very different stability issue than a tall 300 pound guy with 50 pounds of gear.

My point is this, stability is not the end all.  It may feel safe in the beginning, but you are selling yourself short.  The ultimate in stability is a boat.