The settings here are going to be different for each site as it'll depend on the hosting space available and what the site is used for. For example, a photography forum may allow larger uploads and more attachments than other sites.
On my site (A PS3 gaming forum) I have the following settings:
Maximum Attachment File Size (KB):
10000 (to allow sharing of full size photos in threads and this covers most digital camera photo file sizes)
Maximum Attachments per Message:
10
Allowed Attachment File Extensions:
txt
pdf
png
jpg
jpeg
jpe
gif
tif
tiff
I saw no reason to have anything other than picture file types and possibly pdf and txt though no one has actually used these two
Maximum Attachment Image Dimensions:
0 x 0 (i.e. unlimited. The forum scales the photos to fit posts anyway)
Attachment Thumbnail Dimensions:
400 (I thought it was a little small at 100)