Find Linux very laggy?

As Linux is running on top of android it can be quite laggy, to reduce this look at overclocking your CPU, or possibly using a ROM that is lighter (e.g a AOSP ROM) and so leaves more RAM free for apps to use

Set the wrong Screen size?

If you have set the wrong screen size for VNC and wish to change it (and had set it as default) then go to the location of your linux .img file. Here you should find a .config file, open this with a text editor and you will find a line that relates to the screen size. Change it here then just reboot Linux.