My reply is too late but this might be helpful to someone. The simplest way is to check the weston log which will give you all supported modes for each connector(VGA,LVDS and HDMI) below are the log which highlights these.
Additional info: If you have not connected a physical monitor to connector port and port is enabled then it will by default take 1024x768 as preferred.
[09:23:40.144] Output VGA-1 (crtc 59) video modes:
1024x768@60.0, preferred, current, 65.0 MHz
1440x900@59.9, 88.8 MHz
1366x768@59.8, 85.5 MHz
1366x768@60.0, 72.0 MHz
1360x768@60.0, 85.5 MHz
1280x800@59.8, 83.5 MHz
1280x800@59.9, 71.0 MHz
1280x768@59.9, 79.5 MHz
1280x768@60.0, 68.2 MHz
1280x720@60.0, 74.2 MHz
800x600@60.3, 40.0 MHz
800x600@56.2, 36.0 MHz
848x480@60.0, 33.8 MHz
640x480@59.9, 25.2 MHz
[09:23:40.144] Output 'VGA-1' enabled with head(s) VGA-1
[09:23:40.145] Chosen EGL config details:
RGBA bits: 8 8 8 0
swap interval range: 1 - 1
[09:23:40.151] Output HDMI-A-2 (crtc 58) video modes:
1920x1080@60.0, preferred, current, 138.8 MHz
1920x1080@60.0, 148.5 MHz
1920x1080@59.9, 148.4 MHz
1920x1080@60.0, 74.2 MHz
1920x1080@59.9, 74.2 MHz
1920x1080@50.0, 148.5 MHz
1920x1080@50.0, 74.2 MHz
1920x1080@40.0, 92.5 MHz
1280x720@60.0, 74.2 MHz
1280x720@59.9, 74.2 MHz
1280x720@50.0, 74.2 MHz
720x576@50.0, 27.0 MHz
720x480@60.0, 27.0 MHz
720x480@59.9, 27.0 MHz
640x480@60.0, 25.2 MHz
640x480@59.9, 25.2 MHz
[09:23:40.152] Output 'HDMI-A-2' enabled with head(s) HDMI-A-2
[09:23:40.152] Chosen EGL config details:
RGBA bits: 8 8 8 0
swap interval range: 1 - 1
[09:23:40.155] Output LVDS-1 (crtc 56) video modes:
1024x768@60.2, preferred, current, 65.0 MHz
[09:23:40.155] Output 'LVDS-1' enabled with head(s) LVDS-1