The font size adjustment on the Aura One is not as fine-grained as on other devices, this is because while Kobo have increased the minimum and maximim pixel font sizes for the Aura One, they haven't changed the crossover points where the adjustments switch from one-pixel to two-pixel steps and from two-pixel to four-pixel steps.
It is possible to improve this by using the `Custom font sizes` patch, but it might need tweaking if you want to be able to adjust the font size on the Aura One in one-pixel steps, depending on your normal reading font size.
To put this exactly, here are the 55 font sizes that are available on the unpatched Aura One:
Code:
1-px steps: 18,19,20,21,22,
2-px steps: 24,26,28,30,32,34,36,38,40,42,44,46,48,50,
4-px steps: 54,58,62,66,70,74,78,82,86,90,94,98,102,106,110,114,118,122,126,130,134,138,142,146,150,154,158,162,166,170,174,178,182,186,190,194.
and here are the sizes with the `Custom font sizes` patch default settings:
Code:
1-px steps: 18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44
2-px steps: 46,48,50,52,54,56,58,60,62,64,66,68
4-px steps: 72,76,80,84,88,92,96,100,104,108,112,116,120,124,128,132,
(These are the base font sizes selected by the font slider, the actual final size on the screen is also affected by the book's stylesheet.)