While fairly straightforward to adjust, I still miss a step every now and then when I have to adjust this and scratch my head for a bit.  To that end I thought I would post this for my future self :)

How to increase file upload limit in SilverStripe:

Usually it is as simple as adjusting two settings in your php.ini and restarting apache.  (location of php.ini can be seen if you use the phpinfo() function to see the info)

upload_max_filesize = 16M

post_max_size = 16M

It is usually the latter that I forget to increase...

In some rare instances a developer will have overridden the limit manually for a specific upload field.  You can find that by searching your codebase for "->setAllowedMaxFileSize(".

Enjoy!