Hello,
As part of a helpful community I thought I would offer up a little interweb searching... Maybe skip the slab and go for artificial something or another, keep it to pier slabs dotted around the place.
After all these are big sheds at 9m/12m/15m x 7.5m x 2.4m options...
https://grantsheds.com.au/fertiliser-sheds/
Steve
Ps. Might have trouble with enough level ground though