Joerg,

Thanks for bringing that to our attention. We have already updated the css in the the upcoming 4.1 release, but if you want to make the change to your site before the update is released, look for this file 
/media/j2store/css/J4/j2store_admin.css

towards the bottom, find:

.com_j2store .inventory-form-control {width:60px;}

change to:

.com_j2store .inventory-form-control {width:100px;}

Let me know if this works for you :)

Adam