The size of the header is very important if you use it.
For example, in my organization we place toolbars written in Markdown and we could really use a larger size.

I also vote for increased size of channel headers. Our use case is that we display build badges from Jenkins in the header and quite soon exceeds the maximum limit of 1024 characters

I don't think a header on the channel is really that important. I fee that a header is mean't to be read once, or referred too, and that the length or size should be limited.
The people in the channel or joining should read the description for more information no?