@badmonkey see the reply 3 in this topic from Spuds.
Here is what I have in Elk ACP
Max attachment size per post: 9600 KB
Max size per attachment: 5000 KB
Max number of attachments per post: 15
Here are my PHP settings:
upload_max_filesize 512M
post_max_size 1024M
And this is the result I get in Elkarte under post reply
Restrictions: 15 per post, maximum total size 1,048,576 KB, maximum individual size 524,288 KB