I bought this costume in toddler size for my 2 1/2 year old son.  He is probably average in size and the costume fits him without being too big or too small.  We went to a Halloween party and I was able to fit jogging pants and a long sleeve tee under it without it being too tight.  It seems the fabric has a little stretch to it.

To say my son loves this costume is an understatement.  He wears the cape and belt every day, all day, taking it off only for nap and bedtime.  It looks like the cape is attached to the costume from the picture but isn't.  It clasps together in the front with velcro.  My only issue with the costume is the belt.  The first time he wore the costume the belt started to fall apart.  The belt has a front piece that is sewn to a piece of fabric on each side and they tie in the back.  The fabric is barely held on with a piece of thread.  After I sewed it back on, it was fine.  No big deal really for the use it is getting.  I recommend this costume.