I'm currently building a PD shed too. I agree, the rules are ridiculously vague. Anywhere that actual numbers are given, this is "floor" area, so that may give some leeway. In my case I have 29cm thick brick cavity walls, so it's a lot bigger externally.
I haven't seen an official limit in area anywhere other than informal guides. I don't think the official guidance states anything, in which case it appears that you're allowed to build as big as you like on up 50% of your garden, which would be nuts.