This is another area of maths that children meet very early in their lives. Standard units of measurement (eg metres or kilos) are not taught at this stage – it would be pointless as many children are unable to count up to 100 or 1000.
Rather it is about comparing things – objects which are taller, wider, heavier, longer etc than other objects.
Get the vocabulary right at this age and they will have little problems with measurement later.