Ok.
At first you need to change "default engine to use for this project" to "Javascript (a.k.a. HTML5)" in preferences of your template.
After this, when you add the: media -> flash animation, you are able to add optional properties: "Movie Size" to every flash segment and define it as width,hight eg.: 1280,800 (1280(coma)800).
It is very important to change Display Mode in your Learning Object to
"fill window".
When you do this your flash will be bigger.
The size of the browser window I do manage directly at eFront LMS
I hope it is useful.
Cheers.
Thomas