Licensees have reported that they want to specify the Size in the same unit as cube on level.
Looking at the engine, it seems that the Decal Size is currently set as the half size of the Box.
1. Create a Decal Actor on the Level and set the Decal Size.
2. Compared to the Cube on Level, it seems to be double length.
UE cube : 100cm x 100cm x 100cm
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-165942 in the post.