A shorter height makes them more resistant to rotating, but the narrower width makes them more prone. Consider a log on a pond: It's round. Rotates easily. Plank doesn't. More plank like, more stable. Closer to round, less stable.
A building brick is exposed on it's edge, so you want to increase the height faster. Standard brick is easy to hold in one hand. A really large block makes for interesting problems for openings. If you build with the common ~8 x 8 x 16" concrete block, it affects your choice of windows.
The main function of thickness is to not break under the proposed load. Here in Alberta, concrete block pavers are often used for driveways, so it has to be able to withstand car weights. In general, the thinner and larger the block you use, the more solid your sand bed needs to be. An unsupported patch the diameter of the thickness of the block I don't think is a big deal. It has the strength to bridge that. As the unsupported patch gets larger, it will take less of an impact or load to cause the block to fail.
Blocks have a wide variety in thickness.
Patio blocks -- those 2 foot x 2 foot (sometimes 30" for the 2nd dimension) are usually only 1.5" thick. They are frequently bedded on sand, or sometimes on road crush. I don't see them used for driveways.
From the manufacturer's point of view they are balancing the following:
- more material to make = more equipment, more money.
- may be able to use leaner cement ratio, which is cheaper.
- more expensive to ship.
- gives more substantial feel to final customer.
- Edge finishing more difficult. Curves difficult, as there are fewer joints to absorb the variance.
- each block more difficult to move and place.
- final customer has to make better bed for placement.
- Takes more blocks. Placing time below a certain size is per block.
- Fancy shapes easier.
- less stable for a given thickness.
The size around 4x8 is easy to place one handed, so once you have a bed, laying them is fast.
Locally I see pavers that are about 4x8 (your size) about 8x8 (requires two hands to set.) 8x16 (2 hands, but more pattern possibilities) 24x24 patio blocks (takes two people to place easily) 24x30 (ditto) I've also seen hexagonal ones about 10" across, and a triangular one with circular arc edges to lock together.
Locally I see colours being natural (grey-white) dusty pink, and dark grey. I suspect that they are coloured with iron oxide and charcoal black, both being cheap mineral pigments.
Whatever you go with, put an additional patch of them somewhere -- base for a compost bin, splash point for roof runoff, whatever. This way if down the road, you break one, you will have a spare. These things go in fads. Just try to find a matching block from a walk that was done 30 years ago.