The canvas is 1080 × 1920, but you do not get all of it. Instagram stacks the account row and follow button across the top, and the caption, audio credit and action buttons across the bottom. As a working rule, treat roughly the top 14% and the bottom 35% as covered, plus about 6% on each side for the action rail.
That leaves a usable centre band of roughly 950 × 980 px. Put your subject, your burned-in text and your call to action there. Meta publishes the same principle in its design requirements for Stories, which is the closest thing to an authoritative source for where the interface sits. These margins are approximate and Meta adjusts them — the shape is stable, the exact pixels are not, so leave yourself room.