you really have some math to do to get there it would be lots easier to make either a 15" block or an 18" block...3 does not go into 16...but if that is the size you want...using 5 3/4" squares when put together will get you to 15 3/4 finished...so 5 7/8 would get you a bit closer...if you use 6" squares when sewn together...it will be 17 1/2"...might be easier to make it with the 6" squares and then trim it to 16"...but to make a 3-grid block it is lots easier for 3 to go into the finished size.