Digizz,
I work in a small workshop - smaller than what you are suggesting - 10 x 8 foot. It is possible, but clearly the more space you have the easier it is when working on big projects. Also, you will inevitably find you require more space than you *think* you need. I also had restrictions on workshop size - it would simply take up too much of the garden. So, you simply have to make do with what is feasible on occasions.
I don't really keep any wood in the workshop - mostly it seems to live in the guest room! Also, I built a little shed for stuff like the lawnmower, cans of paint, shovels etc. This really makes a difference, stops you getting oil and grease off the mower into the workshop. Due to such severe space restriction I work without natural light - I need the area behnd the windows for shelves - to hold all the bits and bobs you need. To compensate for this, I use 2 sets of flourescents lights - making 4 tubes in total - which keeps it very bright. In the summer, I can open the large door - and as the workshops so small, it does make a difference. Despite seeing lots of people "planning" the layout, I find this has evolved naturally, and changes with the need to accomodate new equipment. I found getting a Tablesaw meant I had to dismantle one of the two workbenches I had in there. Shelving is essential, to maximise the space right up into the roof. The one comprimise I made about storing "other" junk in there, was I have three bikes to squirrel away! I dismantled them completely, and stood them vertical with all wheels, handlebars, seats pedals etc removed. They are actually quite compact. Other than that, I won't let any junk in. Use the loft, or the car boot sale to get rid of "unwanted" stuff.
I'd seriously reccomend the larger of the two sizes you recommend. If (when) you are trying to use either a tablesaw or a bandsaw to rip long stock, you need plenty of space.
However, clearly I get by with a tiny amount of space - see here:
https://www.ukworkshop.co.uk/gallery/cat ... p?cat_id=8
The main things to think about also are electrical power supply, and security - a lot easier to do at the start. I presume you will have a suitable base to stop it rotting?
