I currently live in a 3 bedroom,2bath double wide with a 20 x 40' great room addition which comes to 2300 sq. ft.
No way could I move down to a 14 x 60 single wide from a 3000 sq. ft. home. You are a better person than I. I like my space. I would feel claustrophobic in anything smaller.
As far as the shed, what are the chances of you just adding on a room to the mobile home? That way, it is all connected. Otherwise, I would recommend getting more shed than you think you need, that way you won't run short on space.
As an example, our garden club higher ups decided we needed an 8 x 8 shed to store things in. We peeons tried to explain 6 x 6 was not big enough, that we needed a at least 10 x 10. We no sooner got it built and the powers that be realized we didn't have a big enough space. If you can afford it, go bigger.