Open the model on 0xr.space, press Scale in the dock, then 1:1 Size. The model is shown at the size its file says, and the panel's size line reads it out, for example 45 × 90 × 50 cm. On Meta Quest or an Android phone with ARCore, press View in your space to stand it in the room next to real things. The - and + buttons step the scale by 5%, the slider catches at 25, 50, 75, 100, 125, 150 and 200%, and Fit View brings it back to a comfortable size.
Real size is only as right as the file's units. GLB and glTF are in metres; STL, OBJ and PLY carry no unit, so one is assumed, for example an STL 10 or more units across is read as millimetres. The scale is also held so the longest side stays between 10 cm and 10 m: a smaller part or a bigger building stops at that limit instead of reaching 1:1.
Its file is in millimetres and was read as metres, or the other way round. The size line shows what was assumed; scale it by hand.
No. The longest side is kept to 10 m at most, and 10 cm at least for small parts.
The size line gives the real dimensions, but a screen cannot show the model at 1:1. Use a Quest or an ARCore phone for that.
No. It is read on the device, and nothing is sent to a server unless you create a share link yourself.