Hmm... regular house brick is around 9" x 3" on average although they do come in some other similar sizes.
CMU "cinder" block is around 8" x 16".
But a retaining wall on a tunnel would probably made from larger stone blocks, maybe 12 to 14 inches tall, 24" long. Could be bigger.
So, if you have an O scale ruler, just measure out what looks right to you. If you don't have an O scale ruler....
I model in HO and it is 1/87th scale. It is considered "half-O" so O scale must be around 1/43rd or so. Just do the math of dividng the block size you want by 43... might need to do some decimal conversions and such to make it come out on your ruler.
7 feet? That is up there!