Hey Marcki, within our system you cannot change the size of the FAB icon.
However, in addition to an iframe embed we also have a javascript embed. If you use this and know how to code a little, it opens up much more ability to customize it. For example, one user added custom text above the FAB. On the embed customization page for any Pickaxe, there is a dropdown to select either iframe embed or javascript embed (see screenshot below).
Of course, this is outside of the actual services we provide at Pickaxe, but we do open it up for people via the javascript embed code.