Is Fuller or More Full Correct?

Technically, “full” would be the proper adjective. The comparative “fuller” would be used to compare two things of varying degrees of fullness, and the superlative “fullest” to compare three or more.

Is the word Fuller correct?

The word 'fuller' is a word, and it functions as a comparative adjective. The word full is the base, root word. The suffix, -er, is added to the end.

Is Fuller a word in English?

adjective, fuller, fullest. completely filled; containing all that can be held; filled to utmost capacity: a full cup. complete; entire; maximum: a full supply of food for a three-day hike.