Hello DifficultTree and welcome to the ArtRage forums
That’s a bit strange, Shift+ClickDrag Left/Right should be working for you. Including when resizing the Warp tool.
I have no problems with this method in AR6.
So…
Which OS are you on?
What input method are you using; Pen, mouse, trackpad etc…?
Was it working for you before in AR6?

Unfortunately, as you have found, you can not type in sizes greater than 100% for the Warp tool. So Shift+ClickDrag Left/Right is really the only way to achieve larger tool sizes with it. But as I say it should be working for you