Since a hero with grapple can hold in place any type of monster regardless the size, my gaming group came up with this house rule for Grapple which we thought was quite good.

At the beginning of a monster's turn. The Overlord throws one black dice for each space beyond the first occupied by that monster (that is 0 dice for Skeletons, Beastmen and the like, 3 for a demon for instance and 5 for a dragon). If a blank shows up, the monster manages to break free and can spend movement points that turn.

IMHO it's good idea, but .... in that case give a chance to named monsters ;-)

for named monster you can have +1 or +2 dice than normal (to 5 dice max)

Sounds workable.

Just to mention an alternative approach, some content I'm currently working on uses a new ability called Entrap that increases the movement cost for an enemy figure to move out of an adjacent space by one for each rank, and large figures can ignore one rank of Entrap for each space they occupy beyond the first. Thus, Entrap 5 is basically the same as Grapple against a skeleton, but it only slows down an ogre and does nothing at all to a dragon.

Interesting ideas to consider :-) Many thanks.