Jerichi, your signature is alright and pretty. Shiny hunter Reece, it's alright. j-master, when your spoiler tag is expanded your signature goes way over the height limit. Ash-kid, it's okay. Prinplup, it's alright.
Firekit, it means your signature exceeds 420 pixels and you need to remove something out of or edit your signature in order to put it under the limit.
Here's a walkthrough of how to check if your signature is under, at, or over the limit. If this is well enough, perhaps Ellie will put it somewhere on the first post or something.
First, take a screenshot of your signature. This can be done by pressing the Print Screen button on your keyboard. On a Mac you hit Command + Shift + 3. From there you paste it onto a Paint application. For this tutorial I'm using Paint.net which is free.
Now, look on the bottom right corner and you should see the dimensions of the image that is newly cropped. The number on the left represents the width, and the number on the right represents the height. The one we're focusing on is the one on the right. In this image, I've highlighted where you would find the dimensions and which of the two numbers we need to focus on. The height of my signature is 341 pixels. Since the height limit is 420 pixels, my signature is under the height limit and is therefore acceptable.