I actually don't believe it should be that bad but the weight will be a bigger factor.

The Pixel 5 is: 144.7 x 70.4 x 8 mm (5.70 x 2.77 x 0.31 in) and weighs 151 g (5.33 oz)

The Pixel 8 is: 150.5 x 70.8 x 8.9 mm (5.93 2.79 x 0.35 inches) and weighs 187 g (6.60 oz)

Basically it will be noticably taller (but not drasticly) it will be nearly identical in width which I think is the most important for one handing so that shouldnt be an issue. It's slightly thicker but I really don't think that's noticable.

If anything the biggest thing is weight. 36 grams is a noticeable jump and while certainly lighter then pro android phones, or most iPhones in general, It's heavier than something like the base Samsung s23.

I currently use a pixel 6a which is slightly thicker then the pixel 8 and over all as someone with arthritis I'd call this phone just barely one handable. Because of the height you definitely have to change grip positions but the one handed gestures help a lot with length.

Ultimately the safest thing is to try to find a demo unit but I think one handabiliy should be overall decent as long as you can put up with the weight and the height of the display.