The rulebook entry for Paste of Stickiness states "an object no larger than a weapon". This should be taken to imply that an area of the ground no larger than a weapon (whether that's a long thin line like a two-handed sword or a smaller but wider area like a Shield) can be affected by this substance. In general, a good rule of thumb is no larger than 1-2 feet in diameter without asking a local Marshal about a specific instance.
-Bryan Gregory
ARC
