Just in case this hasn't been figured out yet, there is no hidden stat for increasing the claim radius. You just have to multiply the radius by 2 when inserting the new number into the database. Below is a screenshot:
NOTE:
1) Make sure you adjust the right guild via its respective GuildID
2) Notice how both Radius fields are set to 150? This means that in-game, those claim radius' are set to 75. If you want a 50-radius, set this field to 100, if you want a 100-radius, set the stat to 200.
EDIT: Please edit your subject title to the following so people know the question has been solved when searching the forums for answers:
"[SOLVED] Using SQL to increase Monument Radius?"