We've been struggling for a couple of months to get the image size set to an acceptable size in OSP that ships with IDM 4.6. Modifying the image sizes using the values in the resources/oidp_enduser_custom_resources_en_US.properties file seems to be limited to a range between 72 and 300 pixels in both width and height so in order to get our branding to use a larger image size we had to take the following additional steps.
1. Copy the file /opt/netiq/idm/apps/tomcat/webapps/osp/jsp/inc_common_java.jsp to jsp folder in osp-custom-resource.jar
2. Change these properties in this copied file. All are set to 300 by default. That is why the image size is limited to that. Increase them as per your need
int LOGIN_PRODUCT_IMAGE_WIDTH_MAX = 300;
int LOGIN_PRODUCT_IMAGE_HEIGHT_MAX = 300;
int AUTH_PRODUCT_IMAGE_WIDTH_MAX = 300;
int AUTH_PRODUCT_IMAGE_HEIGHT_MAX = 300;
3. Update the jar file to include this new jsp file
4. Remove the localhost directory, copy this file to tomcat/lib and restart tomcat.