Get some squared paper and make scale models of both plot and summer house (both sizes).Add a scale model of the base with the 18" all the way round on a separate paper. Then you can play around with orientations and clearly see how much space you would have left for for maintenance and access to the rest of the garden.
Remember to take account of where the sun is at the times of day you will use the summerhouse so you will get shade or sun where and when you like them most.
Yes you certainly need 18" between the summerhouse and the fence for maintenance but whether you continue that all the way round is a matter of personal preference and depends on what you do with the area. But remember at some stage you will want access for cleaning/repainting etc but whether you walk on concrete of a beaten soil path is up to you. You may want more hardstanding on the side where the door is - hard to advise without more info.
Whether the larger summerhouse would look too large is not really important to anybody but you. Depends what you want to use it for and whether looks or use matters more. Bear in mind though that having an 8' visual barrier across the garden will restrict the view of the rest and make the garden look much smaller from the house end. This may or may not matter to you!
Hope this helps t get you thinking.

Yorkslass advice is spot on. A scale map is going to help you the most. I usually advise to go to the biggest you can afford but you need to keep it in balance with the remaining space.

Thanks, Yorkslass and Seaburngirl. We went to a garden room company a few days later and bish, bash, bosh we're going to have a 8' x 8' garden room installed at the end of June. Talking to a professional somehow took away all our worries. We've even been round to see an example the company have recently installed in a garden in the next road, which looks lovely. I like the idea of the garden looking smaller - it looked like a long narrow strip when we moved in which is why we divided it into 3 areas. The vegetable garden will be a 'secret garden' when the new summer house appears to be the end of the garden.

so pleased you have found company that is happy to advise you.

Good news that you've been able to make a decision you are happy with and have been able to stop agonising about it! Don't forget to show us a photo or two when its all finished! Having a professional installer should mean an end to all those worrying decisions too.