G'day Faz,

They will, but it will be a close fit lengthways.  Just be mindful to set the tent up fully pegged out with all guy ropes to keep good tension on the canvas at the bag, and you should be ok.
You probably wouldn't get away with your head at the back though.

G'day Rob,

Any mat will be suitable for use with this stretcher.
It's best to pick one with dimensions as close to the stretcher size as possible, most just to help keep the mat in place & stop it slipping, but ultimately if it's a bit bigger there won't be any issues.
Either of the mattresses you've mentioned would be fine!

G'day Dave,

There is no need to use mats with stretcher beds, they are fine enough as a bed in their own right.
You may want to look at adding a mat further down the track if you are heading away in super cold overnight conditions, as that will help provide additional insulation.

G'day Hayley,

No, the W Leg Stretchers are only available in a single size. You could zip two together if you wanted to create a later sleeping space, however, you would have the sidebars running down the centre, and would still need to sleep in your own individual divet.