Hello,
For your needs I suggest you to use/modify your Css code, for customize your product page via your Css,
have a look on this link,
here
.
Note : I advice you to edit your Css file at the end, later that will be easier to find and correct it.
I can suggest this kind of code for example have a 650 px width pop up :
.view-checkout div#sbox-window iframe {
width: 650px;
}
And :
.view-checkout div#sbox-window {
width: 650px;
}
Note : that in your selector, "
.view-checkout" is required in order to only works in your
checkout page !
Without this
ALL your pop up will be concerned, and you don't want this, I think.
To learn more about Css code and their properties, I invite you to read this source,
here
Hope that will help you.
Regards,