The maximum file size is 10mb.

Photos are simple to resize.
For GIFs...
The easy way to make sure your file is under 10mb is to use the Img Play app and when you save it... save it as 10mb.
As you can see me doing in the GIF below. Just select "gif size" instead of "save to photos".

When uploading to your blog, this is what I do.

For some reason uploading the GIF on DPorn/Steemit desktop/computer the GIF will not play. So I upload to the desktop version of Esteem, then I copy and paste the url into my DPorn blog. As you can see me doing in the GIF below.

Mobile DPorn works

You can upload your GIF straight to DPorn/Steemit on your mobile device. That works without any issue for me. Still needs to be under 10mb.

I hope this helps you.