Just a note (from experience gathered):
Majority of render engines (Indigo, Iray, Maxwell, Arnold, Thea, Centileo, Cycles, Vray, RedShift, FurryBall...) specifically, their interactive viewports, render independently of resolution/dimensions set in render settings. Preview is based on the active camera view in relation to IPR window size (which also scales interactively). Only Corona's & Renderman's IPR are locked to dimension settings.
So most likely it's a feature request?
Another possibility would be to use render presets to switch through interactively - for now IPR needs to be restarted.
PS.
Bug?
- If cursor is on the viewport, IPR is restarting indefinitely (with small sizes it stops when cursor is off/out of the 3d viewport).
Probably a plugin conflict.
Simple scene (sun, camera, cube, plane) was originally made for CentiLeo & then rearranged for Corona.
Fresh scene works fine.