At 300DPI, 8.75×11.25 is 2625×3375. For Blank Canvas you can enter these dimensions directly.
For a Grid Collage, as this is larger than a standard size collage, you need to first change your Image Quality setting to Archival (you can find your settings by clicking Hi Username! on the top right toolbar within the editor), and then change the quality on your collage to High (on the top toolbar). You can then drag the edges of the collage until you reach this size by watching the dimensions in the Zoom box down the bottom right of the screen (I’d suggest designing your collage first and then dragging the edges as otherwise the dimensions may change as you change your collage). You may not be able to get the dimensions exact to the pixel, but a few pixels off won’t matter.
Let us know if you have any other questions!