A long tape measure would do the trick. When they say their allotment is so many rods, they actually mean square rods. Allotments are probably the only thing that is still measured in square rods, though by law rents now have to be presented in square metres.

A rod, pole or perch is 5.5 yards or 16'6". This may seem odd, but it fits in with the imperial system of measures which has a factor of 11, ie 1760, the number of yards in a mile, is 160 times 11. There are 4 rods to a chain, 10 chains to a furlong and 8 furlongs to a mile, so 40 rods to the furlong and 320 rods to the mile. Railways are traditionally measured in chains. A chain (22 yards) is also the distance between the wickets on a cricket pitch.

An acre is the area of a plot 4 rods (a chain) wide by a furlong long, or 160 square rods. This comes from the days of ridge and furrow agriculture, where agricultural strips were traditionally 4 rods wide.

10 square rods seems to me to be a huge allotment, much bigger than most individual plots I see in allotment areas. In fact I think it is a bit bigger than my back garden. A nursery, now closed down, in Surrey told me that they were paying about £8,000 a year for their 1 hectare holding. That's 40 times as large as a 10 rod allotment. So if you are only paying £100 for a 10 rod allotment, that's only half the price of a commercial smallholding rent in the home counties. And I bet your allotment has plenty of unused land between the individual plots to provide access, that isn't charged for. Think yourself lucky.