Personnally, I recommend trying on the devices themselves if possible (preview over wifi), export frequently to also try the exported version on the mobile (through a browser personnally).
I'm gonna copy paste a skype post I've made:
"-there is not only one resolution, nor aspect ratio, so I do the following
choose a basic aspect ratio, one you like to work with, I personnaly like 4:3, and also a resolution that correspond (I like 800x600)
Enter it in the "window size" propriety in C2
Right now your game is set at a resolution, however, it won't appear fullscreen on all devices, to do so, you can:
Set fullscreen in browser: Scale inner or Scale outer (scale outer will display more, scale iner will display less)
Use the system expression ViewportLeft(Layer), ViewportRight(Layer), ViewportTop(Layer), ViewportBottom(Layer) to place the elements of the HUD and others, these expression calculates the positions of the left, right, top and bottom border of the screen
use the browser object to detect if the user is on portrait or landscape"
Also there is ot a standard for locking screen inside browser I think, so you cannot be sure it will be locked, however you can still detect the current positio of the device