My son is getting married in September. It is a mid-afternoon wedding and I don't know if it is semi formal or formal. What is the current etiquette for what the length of dresses and suits should be for afternoon weddings? I was thinking of a suit with knee length skirt. Would that be more appropriate than full length? I am plus size but short and tea-length dresses do not look right on me.

I think etiquette would state knee length or tea length. But, anymore, anything goes.
I would say you should wear what is comfortable to you and what you feel you look pretty in.
I think really the only rules that still apply are - no white (anytime) or black (for afternoon).
You will "wow" them more if you are comfortable in your own skin!

Just an aside here, but I don't think you should be trying to "wow" anyone. This is the BRIDE'S day and no one should taking attention away from the bride. I'm not saying you shouldn't try to look attractive, but no need to wow anyone.