It can become very confusing when choosing the right size shed for your garden. Lots of people make the mistake of ordering a shed that turns out to be too small for their gardening needs. You should always sit down and do some measuring before ordering your shed to get the true dimensions of the building. It is always important you don't rush into a decision without first doing some research.
With most shed companies the shed size quoted to you may not be the floor area that is actually available inside.  A garden shed described as 10’ x 8’ (3.030mm x 2.440mm), for instance, will probably give you an internal floor area of around 2912mm x 2312mm). This is because the almost all measurements refer to the exterior of the shed.
You must remember the overhang of the roof of the shed to make sure it fits within your dimensions.
One thing you need to consider is what exactly you're going to keep in your shed. Sheds vary hugely, from narrow bike store buildings to large working workshops. 
Think about the shape of your garden too for example if you only have a small garden then a pent shed against a wall would be a great addition.
The sloped roof of a pent shed will allow you to place the shed right up against a house wall or fence, and maximises the space available inside the shed. If you have a a larger garden and want to make your garden shed a feature of your garden then another option to consider is a apex shed with a more traditional look. This type of shed offers plenty of headroom iternally. Most Sheds are sold by imperial measurements, with the most popular sizes being 6x4