The key to measurement in the early years is to see it as a way of comparing. Year 2 children should continue to make comparisons, saying which is longer, shorter etc. Measuring using a ruler can be introduced in year 2 (5/6 years old), but make sure that a centimetre ruler is used and that when measuring a line start at the correct end ie. start at zero. Many children will ignore this to begin with. When measuring a length always estimate (have a guess first) eg “I think this line is about 10 cm long”.
A metre ruler, or a stick cut to a metre length is also useful (Beware that it does not become a sword!) for measuring longer distances such as the length of the garden. A tape measure can also be used, but once again be careful that you are not using numbers which are too large as children are generally still dealing with numbers up to 20 or whole tens.
Probably the main point of measuring at this stage is for the child to get a good idea of how long a centimetre is and how long a metre is so that they can make sensible estimates.