Time for Trigonometry (aka. how to go Pythagoras)
There is nothing ignorant about asking questions (especially before you plunk down money on a sail).
Using IJPE will give you the planned max luff and foot for your headsail. J is the foot of your headsail if it filled 100% of the triangle formed from the Top of the mast, to the deck (also known as measurement I), then forward to the tack for the headsail and then back to the top of the mast.
So, on a Oday 23-1 with an IJPE of 27.8'/9.6'/24'/9.3', your max luff would be about 29.4 feet, and max foot would be 9.6 feet. These numbers should be the theoretical max Jib for your boat.
You can however go longer on the foot (and that would change the percentage.) For example if the foot of your jib is actually 12 feet long your are running about a 125% jib, which I think was standard on the O'day 23's.
If you buy a used sail, they are often listed by luff and foot, so those measurements above are the starting point for your search (check minney's yacht surplus for numerous used sails too.) I would recommend getting out a tape measure to make totally sure that your rig matches the proposed dimensions above. Manufacturers often changed things during a run, and with owner modifications not to mention just wear and tear, dimensions can be different from proposed.
In short, look for a luff of 29 feet and a foot of around 12 feet and I think you will be happy with the result. And then of course...go sailing!
