System Information
w10 last update, gtx1080 last drivers

Blender Version
Broken: 2.79a
Worked: 2.79

I want to tweak particles with the "hair" type.
In 2.79a, size and random size aren't available in the 'physics" tab (and nowhere else). I have to uncheck "Advanced".
In 2.79, it is available.

Exact steps for others to reproduce the error
Create 2 meshes in Blender 2.79, one very small, one normal. Create new particles system on the biggest one and use the "hair" type. In "Render" tab, use the sub-tab "object" to choose your small object. Check Advanced and go to "physics" settings : you can tweak size and random size.
Save and open in 2.79a. Size and random size aren't available.