(keitai-l) Acceptable graphic sizes by phone

I'd like to setup an autoresponder that mails the map to my house to the
sender of the mail. 

Now I am wondering for mobile users what is a good image size and format
to send? My Au sends 132 x 176 pixels,  24 bits per pixel JPEG which
fits nicely on the screen. How about other phones? 

I notice that much larger images can be received and displayed and the
user can scroll around them to see the whole image. This is not so
useful when someone sends you a portrait but is more useful when the
image is a map.

Is there a rule of thumb for the maximum image size that will be
suitable for all phones? Or should I go crazy and send different images
depending on the phone model?

Stuart