Hi Pawel,
We have checked your query and we would like to inform you that currently, SfImageEditor does not have support to disable the resizing behaviour of crop rectangle. However, the SfImageEditor having various inbuilt cropping ratios, you can choose it from crop toolbar items. Also, you can define your own ratio by using the ToggleCropping() method of SfImageEditor. Once you have chosen the required ratio from the crop toolbar or using the ToggleCropping() method then, the ratio will be maintained while resizing the cropping rectangle. Please find the below user guide documentation for more information about crop support in SfImageEditor.
Regards,
Eswaran