I had same problem on one of my clients store.
try this: skin/common_files/modules/XPayments_Connector/xpc_iframe.tpl
around line 12 try setting height to auto and perhaps remove or just add height according to your need
Code:
<iframe style="width: 100%; border: 0px; height: 0px;" border="0" marginheight="0" marginwidth="0" frameborder="0" class="xpc_iframe" id="xpc_iframe{$payment.paymentid}" name="xpc_iframe{$payment.paymentid}">
Hope that helps