The cost difference between 16” and 24” construction is minimal. And is more of a personal choice, not withstanding building codes. I've seen country side buildings using 2x4 rafters 24” OC on a 12' low pitch roof. That have stood the test of time. My preference is always 16” everywhere. Though I have used 12" OC for floor joists.
I got tired of juggling things around to get my small Ingersoll garden tractor and scissors out. Answer I built this 15' by 16' shed. Studs are 2x4's and rafter are 2x8,, 16” OC. Using 5/8” T11 siding and 5/8” roof sheeting. Insulation in walls and ceiling. Always use 5/8” type X fire-rated drywall. Where possible using 4' by 12' sheets. Installed 240 Volt 60 Amp load. All wiring 12-2 romex. LED lights, and electric door opener. Heated and A/C from a small through the wall unit. Garage door was in my stash 10' wide 8' tall that's insulated. Then 36 steel man door was also in my stash. The build took me two weeks..
Paint and epoxy coated floor a nice touch..

The 14" gear head lathe found a home inside..
Below is a 15' by 30' lean-to I constructed with the same construction as the shed. But has a 240Volt 100Amp load center. Build set me back three weeks of time.

MajicPac Through the wall gas heat and electric AC unit..
