Hi ‌

There is no hard limit on the number of controls you can have on the form therefore I would assume there is no limit to the height of the form.

the only issues I can see would be performance of the form loading.

see this article by  for more information:

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Since this is a commonly visited post, for the visitors..

It seems that on at least Nintex for SharePoint Online, the canvas size is unlimited, however controls are limited to beginning from around 32700 pixels. (signed 16-bit integer size?)