In UI3, click on the 3 dots in upper right corner, go down to 'Help", read about group settings.
You can set up a new group in BI if needed, set it to have the size you want, then when in UI3 bring up that group down at left side drop-down, 'Current Group.'
Additionally, if you want to adjust just 1 or more cameras, in Blue Iris you can go to Camera settings => Video, check the box for 'anamorphic (Force size)' and type in above that the size you want, like 1920 x 1080 or 640 x 480 and click on 'OK'.