It sounds obvious, but in order to reduce the chances of the gates being the wrong size it is vital that the supplied measurements are as accurate as possible. If you are measuring between existing posts or columns, please make sure that you take readings down at ground level and at least 1 nearer the top. Take care to have your tape held level across the driveway when doing so and get someone to help you.
It’s always handy to visualise what the gates are going to look like. A quick sketch can help spot any potential issues such as slopes or walls. Try and draw the gates from the road side with your property in the background.
Take account of any slopes and record these on a plan view (i.e looked at from above). You especially need to find out if the drive surface slopes upwards in the area where the gates are going to open inwards. If you get this wrong, you run the risk of the gates hitting the ground during opening. Laser site levels are generally available from small tool hire facilities and are easy and accurate to use. It’s also a good idea to check to see if any existing columns or posts are upright with a spirit level.