Things to consider….
Need or want? A great question but with many answers and maybe not the best place to start, so let’s look at how we can break it down and then take it one step at a time. What is your shed’s primary function; storage, workspace, leisure space?
Being shed experts we can confirm that you will have as much stuff in your shed as the size of the shed, rarely is a shed ever half empty!
So, if you are replacing a shed start with the size you already have, it would be easier after all to use the existing base. Clear out that existing space repurposing, giving away or throwing away everything that is not needed. Then with what is left you will know if you need larger, the same or smaller?
In the garden shed market the most popular sizes for an average garden are 6ft x 4ft, 7ft x 5ft, 8ft x 6ft and 10ft x 8ft, so this might be a good starting point.
Storage
If it is a new project try and imagine the whole space, mark out a shed size on the lawn and think what could go in there, maybe bring a bike or the lawn mower into that marked out space to get a feel for how it would work and put it in context. If you are planning on sitting or working in the space, get the chair you will be sitting on and maybe mock up a desk.
Be aware of the full height, permitted development rules generally allow shed heights to be up to 2.5 m high when on a boundary and 4m when 2m away from a boundary. 2.5m is as high as a standard ceiling in a new build house and so shopping around for a higher shed will give you significantly more cubic space, great as long as that is space you can usefully use.
Work space
What are you going to be doing? If it is a bit of gardening, think about what time of year you will need it for. Generally springtime is when you need a frost-free space, garden furniture storage is probably only needed during the winter. So it might be that your space can be used for different priorities at different times of the year?
If you’re looking for a space to potter about in with a bench (man cave stuff) then allocate a specific side to that, maybe by segregating a storage area from a bench area with a shelving unit or even a partial internal wall.
Leisure space
If the primary use is to have a garden room, a destination in the garden where you can while away summer days, consider keeping this space completely clear bar the chairs and table. It is not going to be relaxing sitting next to the bikes or lawn mower, so give them their own space; a shed can easily have two rooms or a lean-to section at the back of the shed.
Having your space split into two sections does not reduce the overall cubic space, but does mean you can have a different feel for different areas. There's no reason why you still can’t use the garden room area for storage in winter when you are less likely to be sitting out.