ffmpeg - Draw a box and scale its size with time
Thanks to @Gyan comments I've been able to come up with this solution, in case somebody else is interested:

color=c=red:size=1920x1080,
scale=eval=frame:w='if(gte(t*20,600),600,t*20)':h=20[cl];
[input][cl]overlay=x=0:y=0[finalOutput]


Just note that it is important that you download a current git version of ffmpeg because, from what I understood, previous ones do not support the 't' variable in the scale filter.