Since everyone else is debatine 4x5" vs. 8x102, I'll just jump in to recommend 5x7".
The cameras are bigger and heavier than 4x5", but a lot smaller and lighter than 8x10". With a reducing back they are only slightly heavier than a 4x5" (especially if you compare a 5x7" Shen-Hao to a 4x5" Technika). Most lenses for 4x5" will cover 5x7", so there is very little need for "upgrading". Even old "convertible" Symmar 150mm/f:5.6 cover 5x7" straight on!
Except for very wide lenses, a reducing back poses no problems in use.
5x7" cameras have longer, wider bellows, which make extreme movements and long lenses easier to use.
5x7" contact prints are a lot larger than 4x5", and only slightly smaller than 8x10". This sounds crazy, but is very much my experience.
For the record, I shoot 35mm, MF, 9x12cm, 4x5", 5x7", 13x18cm and 18x24cm. Every size is different, but in LF I find 5x7" (13x18cm) to be an exellent compromise between size and portability.
Now I only have to sell some picures, so I can afford a 5x7" Shen-Hao - it's on top of my wish list!