Don't be discouraged if you've got a bigger phone: the Backbone's maximum extension width is 10.14 inches (25.76 cm). Given that the huge iPhone 15 Pro Max doesn't even reach 7-inches, unless you're using a tablet you should be okay on the size front when using a Backbone One.
My hands on experience with the controller left me impressed with the mechanism of the extension, but its effectiveness does make it tricky to slot your phone in it while also trying to connect it to the charging port. Baring in mind that this was with a 6.1" iPhone 11, you might find it's even easier if you have a smaller phone! It took me a few runs of taking my phone in and out of the device to feel comfortable with handling it, so while your phone will fit in it, patience is key.