Although there is no restriction on racket size, shape or weight there are restrictions for rubber covering the racket.
From ITTF rules and regulations handbook 2017,
Rule 2.4 The Racket
2.4.3 A side of the blade used for striking the ball shall be covered with either
ordinary pimpled rubber, with pimples outwards having a total thickness
including adhesive of not more than 2.0mm, or sandwich rubber, with pimples
inwards or outwards, having a total thickness including adhesive of not more
So, rubber can be 2.0mm or 4.0mm thick depending on type of rubber.
2.4.4 The covering material shall extend up to but not beyond the limits of the
blade, except that the part nearest the handle and gripped by the fingers may
be left uncovered or covered with any material.
Rubber attached to professional table tennis players blades, is slightly bigger than the blade itself is not true because rule clearly mention rubber cannot extend beyond the limits of the blade.
Other source: allabouttabletennis.com