Specifying the image (poster) size for embedded sound files

HI. 
I need to embed many MP3 files in a PDF. I'm using Acrobat Pro, "Rich Media"/"Add Sound" dialogues. I'd like all instances to have a standard sized image of my choosing. 
The process for embedding a sound file requires a box to be drawn on the page, which will contain the image. However, I can't find a way of specifying the exact size of the box, and nothing which tells me the size of the box as I'm drawing it, so it's difficult to get a consistent appearance. 
Any suggestions, please?