1 unit in PlayCanvas is generally considered do be 1m. So in Blender/Maya/3DS Max, take care to set the correct working units. If you choose meters (m) as your working units, make a cube 1x1x1. But if you set your working units to centimeters (cm), make a cube 100x100x100. In PlayCanvas, both will result in a 1x1x1 cube.