I've not built with straw bale, but people around here have (The
Compost Education Center in Victoria, BC for one example - it's a beautiful building and has lasted well).
Climate - do you have dry periods when the outside
cob layer would dry? Here, we're wet 2/3rd of the year, but have a couple months at least of summer drought. England does also, and they've got cob buildings hundreds of years old and straw bale is a variation of that.
Foundation - Our Eco-Village has built with this and I recall that they do a gravel footing followed by shale-like rock slabs for the first foot of building. My recollection is that they made a big point of putting flashing between the rock layer and the cob/bale layer so that they didn't have moisture wicking up from the ground.
Building - due to codes here, "straw bale" is really a version of "timber frame" with bale in-fill, but we're talking bigger buildings than a shed. That said, consider the types of storms you get and what they might do to your shed. A key part of storm survival is making sure things are tied together with bolts/wire/ginormous sewing in the case of some straw bale/rebar etc.
Roof - on my property, due to the "big hills in strategic spots and a few mountains that re-direct the weather", our driving rain most often comes from the east or south-east. If I were building this, I'd want a two foot over-hang in those directions. There is
no law that says your building has to be symmetrical (although Hubby doesn't really believe that). The idea is to figure out how much roof overhang you need to keep most of the rain off the walls, and that's a balance between how high the rocks are from the ground up as well has how large an overhang. Overhangs don't need to be wasted space either - they can be micro-climates, seed hardening area, sitting area, outdoor food prep etc. Planting shrubs in key spots to redirect wind/rain also works.
Alternatively, you just accept that you've got spare bales and not care if they turn into compost. In that situation, I'd build a timber framework, with a foundation and roof built to absorb what nature's likely to throw at it, plop the bails in the spaces to act as "disposable walls", and use it until they need replacing. Personally, I would find the increase in the rodent population not worth it, but it's all about what you want the shed to do for you!