Hello, I've tried anything but finally found a solution in this topic.
1. find your device at /boot/bin directory
2. install Sunxi-tools http://linux-sunxi.org/Sunxi-tools
3. convert binary configuration to fex format (bin2fex /boot/bin/yourdevice.bin /boot/bin/yourdevice.fex)
4. change display resolution in following sections:
[disp_init]
disp_init_enable = 1
disp_mode = 0
screen0_output_type = 3
screen0_output_mode = 5
screen1_output_type = 3
screen1_output_mode = 5
fb0_width = 800
fb0_height = 480
fb1_width = 800
fb1_height = 480
[hdmi_para]
hdmi_used = 1
hdmi_x = 800
hdmi_y = 480
hdmi_power = "vcc-hdmi-18"
hdmi_cts_compatibility = 1
5. convert fex to binary format to apply the changes (fex2bin /boot/bin/yourdevice.fex /boot/bin/yourdevice.bin)
Boot&Enjoy!