There is a size limit of 2 MB for each image you can insert. If you go above this limit your images will result in an error. If you are going for a large infographic this amount is too low. I have created several posts in the past which involved infographics and animated gifs, what @TryNinja has suggested is the trick to create "beautiful infographics" on bitcointalk. If you are working on adobe illustrator try adding up more Artboards and splitting up the infographic. Export them separately and you can them insert them like this.
[img]URL of 1st piece[/img]
[img]URL of 2nd piece[/img]
It will come out as if they are single one big infographic. Check this post as reference:
https://bitcointalk.org/index.php?topic=5181918 , Use the quote button and you will see how I designed it with the BBcode. If you really want to get creative you can mess around with code even further. E.G This code will horizontally link 2 separate images.
[img]URL of 1st piece[/img][img]URL of 2nd piece[/img]
As for the width, it varies what resolution you are targetting. For example if you are targetting the 1080p resolution then 1529 px is your figure this scales well on even 2k resolution... Otherwise, you can use this formula to calculate the appropriate width for the resolution you want to target...
0.7963 * Width of resolution you are targetting
This should give you the width in pixels for the resolution you are targetting. Please do keep in mind if you are going to join two images horizontally with a higher resolution than 1080p then it won't work on the lower resolutions. Your images will keep dropping to the new line. E.G Let's say you are targetting the 1440p resolution and your image width is 2560px, if this was split between two horizontal images, each image's width will be 1280px. The issue is the image right across it will drop on the new line, it won't scale well on the 1080p resolution. However, if it's a single image of 2560px, it will scale well even on 1080p and lower...
Feel free to ask me anything, I will be more than happy to help...