The maximum size for a specific toolbar item is 25MB per item. Files larger than this have the potential to cause Office to crash and for certain workstations to experience problems processing the file. 

If you have a file larger than the permissible size consider using a compression tool to scale down images and videos prior to uploading them on your toolbar. There are many tools available, one you could consider to use is convertio. 

There are no limits to how large your toolbar can be, only to individual items. Do remember that the larger the toolbar, the longer the synchronisation process will be.