Your max capacity with a keggle is probably 14 gallons of boil. So the most you could do is 10 gallon batches anyway. 5 gallon isn't unreasonable for a keggle.
Now, if you were using a broad 20 gallon megapot for five gallon batches, your evaporation rate would be higher than normal. But you could adjust for that in your recipes and still have no problem.
In my opinion - you're on the right track with a pot that will serve you for 5-G and still allow you to knock out a 10-G batch.