We ask for 2 feet (just over 60cm) of free space on the sides and in front of your meter.
This is a rough guideline for the amount of room the engineer will need to complete the job. They need enough space to safely and comfortably get around the meter to remove your current meter and install a new one.
It's hard to know exactly how much space will be needed, depending on your meter set up. It's up to the engineer on the day to determine what's safe.