I'd draw it, even loosely, so that everyone understands the question.
If I understand the question correctly, you need 24.5 (we'll use that number as finished for simplicity) inches per side + the width of the border x 2 for a mitered corner. So, say you need 34.5 inches per side (if that border is going to be 5") to make a mitered corner, so you have to take 34.5 and divide it by however many 4 patches you want and that will tell you how big to make them. Or conversely, you can take that 34.5" and divide it by the size and it will give you how many blocks it will take. But since you need an even number of squares, you almost have to use the first method.
If you are going to butt the corners, you will have to do the same calculation, but you will have 2 measurements do deal with. Roughly speaking, the finished size of your 4 patch + triangle is going to be 5". You'll have 5 going one way and 7 going the other.
Just get out your calculator and start punching buttons until you get the exact finished size you need and then add your seam allowance. Using the calculator, you'll be calculating in 10ths, which will have to be converted to 8ths.
good luck
bkay