The maximum size of the canvas depends on what Balsamiq product you are using.
Balsamiq Wireframes Based Products
In our Balsamiq Wireframes products (like Balsamiq Cloud), the canvas is 10000x10000 pixels.
Balsamiq Mockups (2 and 3) Based Products.
The maximum size for each wireframe canvas in our Balsamiq Mockups products is 4056x4056 pixels. This is the maximum canvas size that Adobe AIR apps allow.
Our apps automatically adjusts the canvas size to adapt to your content. If your wireframe bumps against the maximum canvas size, you could try using a breakline to indicate continuation. This is something that designers often do in wireframe specs. Mockups supports breaklines in the Browser, Rectangle, Geometric Shape and Horizontal Rule controls.