The phrase "to check the size" means to verify or determine the measurement or dimensions of something.
Sentences with «to check the size»
Please check the size chart and order a size up if you are between sizes. (deadlyisthefemale.com)
When buying bedding for adjustable beds, whether regular cotton or organic cotton, double check the size of the sheets to ensure they are not too narrow. (pregnancy-info.net)
The leash is available in several sizes so please check the size guide before ordering. (shop.akc.org)