I want to say those big long bolts are a 20mm. I know it's not over that, the upper control arms are 21mm, I had to buy that one. lol I know the bolt heads are recessed up in there, can't get anything but a socket up in there.

Most bolt heads are measure from one flat side to the other across the middle. Use a tape measure or a ruler to check. The measurement will be slightly under or slightly over the size you need. Round the number up or down to the nearest spanner size. You might need to use an adjustable wrench for an 8 sided nut.