Yeah, just measure the width of the wall then the height "floor to ceiling.
If you are measuring in just inches then say the wall is 170" wide by 108" high, devide both measurments by 39.370 and that will give you the measument in meters.
170 \ 39.370 = 4.318 m
108 \ 39.370 = 2.743 m
Now times the two together to get the square meter of the wall...
4.318 x 2.743 = 11.844 sqm
Then add on atleast 10% for wastage, cuts and cockups
If Im measuring up a room with a fairly standard sized window then I still just measure the whole wall since you'll use quite a few tiles cutting into the window, but if the windows very big then I would break up the wall into units to measure to get an exact square meter amount, then add on a bit for the window cuts.
For a bathroom always use a good microban sealant and grout too
