Spam is an abstract word and can be understanded differently.
But in the last few months I found a tendention of spamming signature slot in geometric progression.
I see more and more posts containing 1-2 words and then i need to scroll half of the page to the next post because of previous post's signature.
I understand that you want to advertise your servers, and super-duper custom stuff. But please - do it in appropiate way so that it does look & feel good.

I will bring you a very good example of what I am talking. Check the attachment for image.
I did expend the browser to "Full screen" and i could capture only 2 posts containing 1-2 lines each!
This becomes obvious when we ask ourselves - why they are doing so.
The answer is: randomly do non-informative and useless posts by leaving lots of spam attached as signature.
And when i kindly asked+warned this person he started to be cocky telling that "if others do, i can too".

Since today I will personally hunt such seflish people and fight them their tactics: genociding their posts and deleting server advertising topics.
The server should be kept clear. Reformation has come 

If you ask me what size of signature should be? Not bigger than your posts. But in average try keeping Y size of ~200 pixels max.

what im doing is spam?

you can advertise for your servers in your signature, but try small ads, like i did

now you know the truth.
I don't have any single server and that's pain looking at all those "cool and pretty" signatures around.
It is like going aroind in the park for children without a child. 

I find mine signature small enough to be scrolled in a glance.
If you're defending yourself then i didn't mean yours being out of range.
If defending others - those people do nothing except posting info about their servers instead of helping others/giving back something to community.
When i say posting info i mean that they post 1 line of text and then it is half of the page of his "cool&pretty" signature.

I'd vote for a limited size on sigs.

Like locking the size to a preferred one.

Sadly I don't have access currently into it.

Also if that doesn't work then set it as a rule into the Forum Rules.

How can i post a image on signature??

are my sigs ok ??