Hello Tomáš Čeleda,
There is no out of the box way to achieve what you're after. Would you care to elaborate what is the scenario that requires you to save a report as png image with indefinite height?
All the best,
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items