If you have already run advcfg.exe, then you're fine, but you should run advv.exe with the -advmenuv parameter (read the file advv.txt in the advmenu .zip file for instructions) to set up resolutions for advmenu.
Your section option is to cut and paste your resolutions from advmame.rc into advmenu.rc so that it can use the same resolutions.
The following is an example only, and should not necessarily be pasted into your advmenu.rc file. Your monitor settings may be different and you should use the values you find in your own advmame.rc file, assuming that you have it configured correctly...
...
device_video_format 15750 0.645161 0.16129 0.0645161 0.129032 0.916031 0.0114504 0.0114504 0.0610687
device_video_pclock 5-80
device_video_hclock 15.75
device_video_vclock 50-60
device_video_modeline standard_384x288 9.68352 384 496 536 616 288 299 303 317 -hsync -vsync
device_video_modeline Custom_240x248x8 6.92999 240 336 360 440 248 251 254 271 -hsync -vsync
device_video_modeline Custom_256x242x8 6.93 256 344 376 440 242 244 247 261 -hsync -vsync
device_video_modeline standard_vga_328x240 8.30015 328 424 464 528 240 243 246 262 -hsync -vsync
device_video_modeline ntsc_320x240 8.05581 320 416 440 512 240 244 247 260 -hsync -vsync
...
etc.
Copy and past the device_video_format line, device_video_pclock, etc. and then any device_video_modeline values that you want to be able to use in AdvMenu. You only really need one modeline, once you find the one that works best for you and looks the best on your monitor.
To switch between resolutions, change the parameter of the following line in advmenu.rc. You only need the "X" value, AdvMenu will take care of the rest.
video_size 512