If you know the native aspect ratio of the sensor you can work it out. For example if it's a 4:3 sensor then 12MP is about 4,000 x 3,000 pixels (I say "about" because the 12MP figure will be approximate), and with a 1.4 micron pixel pitch that's 3,000 * 1.4e-3 mm = 4.2mm wide and 4,000 * 1.4e-mm = 5.6 mm tall - or vice-versa depending on which edge you mean by "width". If it's 16:9 then 12 MP is more like 4,600 x 2,600 pixels, which would give dimensions of 6.44 x 3.64 mm. If you want more precise than that you'll need to look at the images and work out exactly how many pixels there are in each direction.
If you look at the different aspect ratio options for the camera the one with the highest pixel count will be the one using the full sensor (I think 4:3 is most likely, but not having an s8 I don't know for sure).
As for 1/2.55", that type of thing is usually described as the diagonal dimension of the sensor, but frankly
I'm not sure that's completely accurate. As a cross-check, GSMArena describes my s21's main camera as 1/1.76", 1.8micron, 12 MP. The images are 4032 x 3024 pixels (12.1 MP), so based on a 1.8 micron pixel pitch that's 7.26 x 5.44 mm^2 and a diagonal of 9.07 mm, whereas 1/1.76" = 14.4 mm. Those are clearly inconsistent, but correspond quite well to what the linked DPReview article says for a 1/1.8" sensor, so my conclusion is that the common statement that 1/x" = sensor diagonal is not actually true.
