The row name header column width is set to auto in uifigures.
You could find a possible resolution to your case in following link: https://www.mathworks.com/matlabcentral/answers/307334-resize-rowname-column-width-of-uitable-in-matlab
Other work around is to not create the row header column and use the 1st column in the uifigure as a header column and give custom input, where width can be varied using 'columnwidth' property.