I don't know if this has been asked before. Is there any possibility of getting the ability to set my own number of speed-dial short's? At the moment I am restricted to 5 across and there is no way to set the sizes either. In Firefox I have 7 across and 6 down and would like to be able to set Vivaldi the same. For me at least this makes for a tidier looking page. This would greatly enhance usage for me and I think others might also enjoy this as well.
Probably this thread is of some interest to you. At the 11th of November the answer to your question was there not to be such a feature at the moment. Now on Fedora 23 with Vivaldi 1.0.344.24 (Beta) dev (64-bit) I still don't see this kind of setting.
If you follow the first post of the thread RJules3 linked you can modify the SD with CSS.