It's not possible to set custom resolutions for web sessions higher than 4096 x 4096. This limit should be increased to 8192 x 8192.

NoMachine will transparently tune session parameters to support the requested resolution, for example by switching the video codec if the one in use has frame size constraints (e.g. H.264 is limited to 4096 x 4096). 

Please note that when connected to a physical desktop a custom resolution could be ignored if it doesn't match any of the available resolutions supported by the remote graphic card.