How do I find out the gb of a phone from the imei number?

to find out the storage capacity of your device, go to settings>general>about>capacity.

Call Apple and ask. I assume you don't have the phone in your possession, where you could just go to Settings/General/About and see it displayed.

Those websites are notoriously unreliable. Many of them exist to steal you identity or some information about you. If you entered any personal information (in addition to a phone IMEI) be prepared for secondary consequences. Even entering an IMEI carries some risk, as anyone who knows the IMEI can cancel your phone or blacklist it.

Not to mention that the information they provide is frequently wrong.