You may want to change the height/width of each lane of the swim lane diagram. At the moment, the option to adjust each lane’s width/height is not available. But, you can adjust the ratio of the widths of swimlanes compared to each other. You can see the difference on the swimlane diagram below.
You can modify a lane by double clicking the Swim lane diagram and adding a hashtag (#) with the ratio after the text. Refer to the section A of the attached image
As per the image, current swimlane ratio is 1:2:1. Since the default ratio is 1, it’s not compulsory to add the ratio for Lane 1 and Lane 3.
Or you can select the swim lane and go to properties and specify the width ratio in the ‘swim data’ box.
Refer the image.
That’s it. You are good to go now. You can see the difference between the lanes A,B and C with the applied ratios 1:1:2 in the image below.