Yes, you can customize the app icon size in kiosk mode. All you have to do is, while setting up a single or multi-app kiosk policy, choose ‘Launcher’ under ‘Android Kiosk Mode’. Under Customizations, kindly enable ‘customize kiosk launcher’, and choose the ‘icon size’.
The ‘Icon size’ will have four options to choose from. If the option ‘custom’ is chosen, you can set the size of the icon manually.