Actually, threshold voltage may be affected by narrow-width and narrow-length effects.
For short channels, the threshold voltage is reduced due to reduced gate control because of the proximity of the S/D regions to the channel.
For narrow channels, threshold voltage increases due to the fact that the fringing fields from the gate raises the surface potential at the edges of the channel.
Hope it helps.
In level 2 equ. for Vt there is no term for L and W but if you look into the higher models of MOS like level 49 or BSIM3 equ there you can see the terms of Weff and Leff, which effects Vt inversly.