You can change size of mashup video by following below steps:
Open your root site using Sharepoint designer.
Go to Style Library>XSL Style Sheets>Edit PKSMashupVideo.xsl.
Serach for Height="360px" Width="480px"".
Update height and width as per your requirment.
Be sure that height and width of mashup webpart deployed should be changed if needed to accomadate updated player.
Hope this helps.