[crh75]

I find seed potatoes of early varieties tend to be much larger then the late varieties.  Therefore, I would assume that larger seed potatoes would help the plant grow quickly.  As this is not an issue with maincrop, small seeds will be fine.

[allotmentor]

Thanks crh75, I guess that makes sense that it gives earlies a head start that maincrops don't need.

[azubah]

My feet are 9 inches long, in my allotment shoes, so I am often seen walking heel to toe in a straight line, and nearly falling over in the process. It is a good way to measure if you keep forgetting to take a tape measure.
Also from tip of middle finger to elbow is 15" in my case. Good old fashioned measurements from the days before tape measures were invented.