Is there a way to adjust the painting area?

[Juicy]

Yes, It is probably because the original images were axial and the coronal and saggital slices have lower resolution because of the larger slice thickness. You can resample your volume to be of higher resolution using the “crop volume” module. To do this, place and ROI on the volume, then select isotropic spacing in the crop volume module to make the voxels into cube shapes and change the scaling if you want to make the voxel size smaller or larger. You can also edit the resolution of the segment itself in the segment editor. See further instructions here