It depends on many factors, mainly on current, type of cable (Cu,Al or other) and surrounding. You did not mentioned if this 25 meters are from battery to converter, or from converter to notebook. You can get small voltage drop at the end of cable (due its conductivity). In output you have 90W/19V=4,8A, in input you get litle more than 90W/12V=7,5A. Get 15A for safety, and from properties of cables you can read it dimension. For copper cable maximum current 15A is for about 1,5mm
2. For AWG cable you can get properties from
wiki page.