I downloaded some patterns from www.thequiltercommunity.com and the block calls for a 3-7/8" square cut in half. Does anyone know where I can find this size?

You have to cut them yourself from yardage. Many times people just cut 4" blocks and then cut them down to size one they are sewn into the desired form for a quilt.

It's because you're going to end up making half square triangles that will end up being 3 1/2". Since you always have to trim them up anyway, it's much easier just to cut 4" squares, mark a diagonal line on the back, put the two you need faces together and sew 1/4" on both sides of your line. Then just cut on your line, press them open and trim to 3 1/2" square.

I too cut a 4" square and cut to size later

I just read in a book called "Stash and Splash" that if you cut the square 4 inches and use a regular presser foot to sew on both sides of the diagonal line, you will have a 31/2 square without having to cut down to size. On almost all sewing machines the presser foot measures 3/8 inch from needle to edge of the foot so you are making a larger seam to compensate for the larger square. How about that! Rita

Cool. You may not have to cut down to size, but there are still the dog ears to trim off, right?

I was told by my quilter, who has been published 4-5 times in the last two years, to just cut it 4" then cut the diagonal. She said lots of well known quilters do it this way. She has quilted (longarmed) for many well known quilters. Hope this doesn't cause too much controversy.

I alway round up the the next number when given a number with 7/8 because it is so much easier when making HST and you can actually get them squared up to exactly the right size.

mltquilt

Well, I just feel like an absolute fool!!! Here I've been carefully finding the 7/8" line and cutting those squares, and let me tell you my eyes aren't getting any younger. I never gave it a thought to just cut 4" and cut it down. LOL Guess I was thinking why do it the easy way when you can do it the hard way...

you can cut your squares 4" then after making your 1/2 square triangle square it up to measure 3 1/2" that would be the size of the 1/2 square triangle using a 3 7/8" square.
anytime you want to make a 1/2 square triangle you take the size of the finished block, add 3/8" then add the 1/2" Seam allowance. in otherwords, if you want a (finished size) 1/2 square triangle block to measure 3" you start by adding 3/8" then add 1/2" seam allowance (which is 1/4" on each side=1/2")...so you start with (2) 3 7/8" squares (or round up to 4") draw a diagonal line across one of the squares, stitch 1/4" on each side of the lines, cut on the line, flip open and press...2 - 3 1/2" 1/2 square triangles completed.