Usually I keep my signatures below 500px wide and 300px tall. Generally its best to follow the majority, not the minority of people that have obnoxiously large signatures that will most likely get removed by the asims as will.
TriNebulon News: Y18 D30 - Y19 D170 RIP :(
Voted worst art team member of Y19
When i removed yours, it was causing a one-line post to take up my entire screen. I would echo Ulrike's 500x300 suggestion as a nice maximum and add that if you see larger signatures feel free to rat them out via DM. I only change signatures if I notice them.
To answer the question as asked: the limit is largely at the discretion of the staff member evaluating your signature, but we make up for this by not hunting down abusers or enforcing a set size.