I am presuming you are running XaAES right; You should edit the XaAES.cnf file in order to get the desired resolution, it is thoroughly but not very well explained in the XaAES.cnf file and you can also find online documentation for XaAES at
xaaes.atariforge.net
One piece of advice, lose XaAES's eye candy. It will slow it down a bit and from your previous posts...:) And I believe for X to be installed you need an fpu? Please correct if I am mistaken here...