Hi,
In the
CMS Desk -> Tools -> Forums -> Edit forum group -> Forums -> Edit forum -> General you can configure
Attachment max. file size (kB) property, so you will not allow users to upload big files.
You may configure the max display size in the forum directly in the property
Attachment image maximal side size of the forum group web part, so that images will be displayed according to your configuration. This setting is available in the Tools as well.
In the current version is resizing of uploaded physical files not supported (the upload configuration in the Site Manager is not applied on forum attachment). I will create a requirement to add this functionality to one of next versions.
As for your second question, you can change the setting. If the file will not be found in the file system, Kentico will select data from the database and copy file to the file system. Then it will be available in the file system and database in the same time.
Best regards,
Ivana Tomanickova