In OBS, you can right click on a source, hover your mouse over "Transform", and select the "Edit Transform" option (or Ctrl + E). This option brings up a menu in which you can change the source's position, rotation, size, positional alignment, bounding box type, alignment in the bounding box, bounding box size, and crop. It basically gives you full control over a source and allows you to resize and reposition it without simply clicking and dragging with your mouse, which is finicky and inaccurate.
Right now, Streamlabs OBS doesn't offer this precise level of control. I understand that it is still in beta, which means there will be issues and that this could possibly already be planned for the full release, but this feature is invaluable to some. It would be a shame if it was never implemented in Streamlabs OBS.
Currently, you can do a workaround by creating your scenes and sources in OBS and then importing them into Streamlabs OBS, but that's rather clunky and unintuitive, to say the least, to do every time you want to make a precise change.