Looks like we were using size*size before, and now it is just size.
(i.e. to make the particles appear in the same position, a size of 0.5 in 2017.3 has to be 0.25 in 2018.1, a size of 2 in 2017.3 has to be 4 in 2018.1, etc.)

Repro steps:
1. Open Scene1 from the attached project in 2017.3 (I tried in b10)
2. Observe position of particles
3. Open Scene1 from the attached project in latest trunk (I tried 2018.1.0a5)
4. Observe the different position of the particles.

Notes:
- Seems to be Mesh mode specific; 2018.1 has both modes behave identical, while 2017.3 meshes were receiving a different offset from billboards. So this might be a fix and expected behavior.
- Regression, introduced in 2018.1