Minimum dimensions of bike storage shed
Width: If you hang the bikes on the wall, either "head to toe" or at staggered heights so that you can interleave the handlebars, then the width requirement is only slightly greater than 2x your handlebar width. Call it 60". 

Depth: If the wall opposite the hanging wall opens like a barn door, then you don't need to be able to fit your body in the shed, and it only needs to be as deep as the bikes are tall, so that's whatever your seat height is above the ground, plus some clearance. Maybe 36".

Height: Length of bikes, end to end, plus some margin. Maybe 60"? Unless you're hanging them at staggered heights, in which case it's probably more like 75". And you'll want a little bit of a pitch to the roof so it sheds rainwater, so a few inches higher at one side.

These are all inside dimensions. If you're using 2x4s for framing, add 7" in each direction, plus the thickness of whatever your cladding is (plywood, chipboard, clapboard, etc). And don't trust some rando on the Internet--measure this out carefully before you start cutting materials.

I don't know how tight a space you're trying to fit this shed into, but you might want to consider making it a little bigger than this. Plywood comes in 4'x8' sheets, and every cut you save makes the project simpler. Plus you can leave room for your N+1 bike.

Making a shed that they can roll straight into will be easier to use day-to-day, but the width will need to be the total width of all the bikes at their widest point, plus some margin, so that handlebars don't interfere with each other.