Подправьте конфиг lts.conf .
Sudo vi /var/lib/tftpboot/ltsp/i386/lts.conf
Вот пример моего.Разрешение указывается в функции X_MODE_0
[default]
LDM_THEME=ubuntu
# LANG=en_US.UTF-8
LANGUAGE=ru_RU.UTF-8
LDM_LANGUAGE=ru_RU.UTF-8
# LDM_SESSION="gnome-session --session=gnome-fallback"
LDM_SESSION="gnome-session --session=ubuntu-2d"
LDM_GUESTLOGIN = True
#LDM_XSESSION ="gnome-session --session=ubuntu"
#LDM_XSESSION =classic-gnome-session
#LDM_XSESSION="gnome-session --session=classic-gnome"
XkbLayout=us,ru(winkeys)
XkbOptions="grp:alt_shift_toggle,grp_led:scroll"
LDM_USERNAME = vector-plus-plus
LDM_PASSWORD = vector
SOUND=True
X_COLOR_DEPTH = 16
X_MODE_0 = 1440x900
LOCALDEV=True