Try Editing the Sizes (Select dropdown arrow of the legend you screnshotted above) There you can choose to set the sizes from Automatic (default) to a range. I think that may get you to where you're trying to go but setting your own Scale Band.
You can also Click the size pill on your marks card and adjust the scale there and that may help you show more range of sizes as it has a bigger size range to work with if you have the space in your visualization.
Hope this helps.
I knew it would be something simple that i just couldn't find! Thanks for the quick reply!!