Reproduction steps:
1. Open project attached (Test.zip)
2. Open scene "TestScene2"
3. Press play.

Expected result: Objects and their particles will be scaled evenly.
Actual result: Particle length scaled twice.

This issue is related to https://fogbugz.unity3d.com/f/cases/804772/ changes made in 5.5.0b2.

Reproduced on: 5.5.0p2, 5.6.0b1.
Worked fine in: 5.4.3p4, 5.3.7p2.
Regression found in: 5.5.0b2.

Note: consider, that a speed scale and a length scale are coefficients, and shouldn’t be scaled. They affect resulting size of a stretched billboard on different scales proportionally.
In order to achieve appropriate scaling of whole particle system, only physical values (speed, size etc.) should be scaled. Now, since we increase both speed scale and speed itself, it is not possible to scale objects properly, without adjusting speed scale values.