It will fit in 640x260 with room to spare, but I think you are not showing the iframe version, the iframe version will only show when you append /iframe to the end of the Invoice URL received back from the Invoice API.

You will have to enable the timer in your BIPS merchant account. Simply select the check box called "Convert bitcoins and transfer to my bank account", this will enable the timer, so customers are required to pay within 15 minutes for BIPS to guarantee the rate for a merchant.