The best way to get an idea of sizing is to physically stake out the size you were thinking of, where you were thinking the workshop would go - as it's easy to see first hand whether the size works or not.
Once you know the size and what key options you must have, then it's a matter of what styles work within these parameters. With this filtered selection of available styles - its all about personal taste and preference.
Another important factor in functionality is keeping things organized and tidy. Give this some thought in the design stage as it's easy to build things in for storage now, rather than after it's built. Read some of the great tips on organization in the storage shed section.
nless you literally live in the middle of nowhere, or you're building a shed under 100Sq. Ft. you're going to need a building permit. Anything over the size of 100Sq. Ft. requires you to check with your local building permit office and apply for a building permit, and this applies almost everywhere with the exception of those building the shed where no one cares, or will see it and complain or report you to a local permit office.