Display Size upon first time login

hi. i have used airplay recently to connect with TV for the presentation. FYI my current dispaly resolution is 1680 x 1050 but every time i on my laptop, the display size upon the login is very big like im using 1021x640 resolution and then after i login then it will back to normal setting 1680 x 1050. please advice how to make sure the display size is always 1680 x 1050 even while login.

thanks