Click on the aspect ratio button you want to set. In the case of an aspect ratio greater than 1, the script fixes the width and changes the height. Whereas if the aspect ratio is smaller than 1, the width changes while the height is fixed.
Reset image resolution by clicking on one of the buttons on the second row.

Configuration

Aspect ratios can be defined in the /sd-webui-ar/aspect_ratios.txt file. The file is pre-populated with the most common values

1:1, 1.0
3:2, 1.5
4:3, 1.333
16:9, 1.777
# 6:13, 0.461538
# 9:16, 0.5625
# 3:5, 0.6
# 2:3, 0.666
# 19:16, 1.19 # fox movietone
# 5:4, 1.25 # medium format photo
# 11:8, 1.375 # academy standard
# IMAX, 1.43
# 14:9, 1.56
# 16:10, 1.6
# 𝜑, 1.6180 # golden ratio
# 5:3, 1.6666 # super 16mm
# 1.85, 1.85 # US widescreen cinema
# DCI, 1.9 # digital imax
# 2:1, 2.0 # univisium
# 70mm, 2.2
# 21:9, 2.370 # cinematic wide screen
# δ, 2.414 # silver ratio
# UPV70, 2.76 # ultra panavision 70
# 32:9, 3.6 # ultra wide screen
# PV, 4.0 # polyvision

Note the # marking the line as a comment, i.e. the extension is not reading that line. To use a custom value, un-comment the relative line by removing the starting #. A custom aspect ratio is defined as button-label, aspect-ratio-value # comment. The aspect-ratio-value must be a number (either float or int) while the # comment is optional.

Resolutions are defined inside resolutions.txt file. By default this reads

1, 512, 512 # 1:1 square
2, 768, 512 # 3:2 landscape
3, 403, 716 # 9:16 portrait

The format to be used is button-label, width, height, # optional comment. As before, lines starting with # will be ignored.