If you are buying the optics complete then build as big as you want to lug around. However if you are thinking of grinding, polishing and figuring your own mirror then I would highly recommend starting with an 8" f7 as your first telescope mirror.
Many hours go into grinding and polishing and the smaller mirrors can be easier (at least quicker) to correct if you get it wrong. It is very easy to get it wrong but also very satisfying to see a nice Foucault, Ronchi and star test when you to get it right. The skills you will learn with your smaller mirror can then be applied to a larger mirror to some degree if you feel you want to make larger one.
Willmann Bell sell mirror kits with everything you need. An 8" mirror is normally ground with an 8" tool. The larger mirrors also require smaller tools to correct for localised errors although there are many different techniques.
Have a look at
http://www.willbell.com/tm/index.htm for books. If you are doing your own mirror I highly recommend Texereau and Leclaire.
For mirror kits look at
http://www.willbell.com/ATMSupplies/ATM_Supplies.htm
If you want to build a large truss Dob then Kriege & Berry's book is excellent but Berry's own book is quite good for tube designs of various sizes.