The main canvas size is 3,600 x 3,600 points. When output as vectors, the artwork is completely scalable, and the bounding box of the artwork is used as the final dimensions of the exported file.
When output as an image, the artwork is rendered based on the current zoom level, and the exported file always has dimensions 2048 x 1536 px.
Hope that helps,
While the draw app has had a fixed size in the past, there are many new presets in the current version of Draw.
If you'd like to create your own, you can make them (limited to the size your device can maintain) 4096 px squared is on my current iPad. The max value of canvas will vary depending on the resolution you want to work at.