--- a/hw/vga_int.h
+++ b/hw/vga_int.h
@@ -31,8 +31,8 @@

-#define VBE_DISPI_MAX_XRES              1600
-#define VBE_DISPI_MAX_YRES              1200
+#define VBE_DISPI_MAX_XRES              2560
+#define VBE_DISPI_MAX_YRES              1600

The vgabios (both lgpl'ed and seavgabios) filters the mode list by
available memory anyway, so there is no need to keep those low enougth
that the xmax * ymax * 32bpp fits into vga memory.  And when the vgamem
becomes configurable (see below) this will be moot anyway.