Not quite sure if I'm going to answer your question here but, ideally the Rams want a guy who's got size, speed and ability to run a complete route tree at X.
OF COURSE SO DOES EVERYONE.
AJ, while big can do it all vs a guy like Metcalf who has size and speed but can only run a limited route tree of gos, comebacks, curls, slants or a guy like Keon Coleman who has size but lacks speed and agility so you just have to hope he wins contested catches all the time.
In the past Rams have prioritized speed and complete route running over size at X and there are times where the size of a player makes it harder for them to have the agility and ability to cut well and run routes at varying speeds thereby limiting their ability to play in a timing based offensive system like ours.
We'd obviously like a guy who can do it all like AJ, Sutton, Chase, Tet, etc... but given the restraints of getting a unicorn at X, the Rams have prioritized speed and agility over size in the past with a WR at X.
Question is: who in the draft may be a good fit at X. Tate and Tyson would be quite good fits it seems from just a cursory watch. Some people like Boston, he could be a fit but I have some concerns about his agility and ability to run a full route tree with us.