I'm not familiar with that brand, but several manufacturers offer heavy duty or high output variants. The length limit for a 3/4" circuit would depend on the flow, which would in turn depend on the ∆T for the loop.
40,000 btu on 3/4" copper. So if normal baseboard is around 600 btu/ ft that is around 66/67Ft.
You could go more but your output will not be 600/ foot after the 67'.
Typically high output is 750/ ft. (ie.. Sterling Senior)
That puts you in the 53' range on 3/4" copper