How to trim a four patch block
 

I am needing to trim a 5 1/2" four patch block to 2 1/2". Need your help. I cannot seem to figure it out.

If you are short of fabric, I would unsew those blocks and save the pieces.

However, if you have plenty of fabric available, cut 1-1/2 inches off each side.

1.5 + 2.5 + 1.5 = 5.0

Half of 2 1/2” is 1 1/4”, so I would make a cut 1 1/4” away from the center line on each side, then rotate the block 90 degrees and do it again in the other direction. If I’m understanding that you want to made currently large blocks smaller.

As stated above, you want to figure half the size of your trimmed block and trim with your center at that distance and your horizontal and vertical seams lined up with lines on your ruler. Trim 2 sides, then rotate 180° and trim the other 2 sides.

You’re trimming a lot off but this method will work for whatever size unit you want to make.

I've done this, with a 2.5" ruler, mark the center of the ruler to put on the center of the block.
Alternatively you could start at the corner and make them all improv which makes an interesting quilt.

I would get a 2 1/2" square ruler and center it. That sure is a small block. Sewn to another block it will be a 2" finished four patch.

Ditto what onebyone said. Yes you have to spend for a 2.5 inch square ruler, but it will save you much time in not having to second guess your measuring, plus you will have an easy check that your square is centered and perfect when you cut. Having a ruler that is the exact size square of your goal square is so convenient.