I'm building a form which requires us to store around 2200 characters in barcodes. I understand that the limit for barcode capacity is theoretically 2710 characters, however no matter how large I set the barcode to, I'm unable to store more than about 900 characters in it. We have a physical limit for the placement of the barcodes in a space of 105mm x 90mm.
I've tried with barcodes of varying size (105mm x 45mm) and (190mm x 177mm) but am still not able to get anywhere near the max.
I tested using characters from the same character set, so that we didn't lose codewords whilst switching character sets, but it didn't seem to help.
I put together a presentation some time ago on this topic. Although dated, it will probably be helpful to you: http://my.adobe.acrobat.com/barcodedformselearningpdf417bestpractices.
In regards to the capacity of 2710 characters in a PDF417, this is really only true in the original specification document. If you stored only upper-case characters with NO error correction at all, no special characters, etc, you could get close to this. However, in real-life with error correction and proper sizing so you can actually decode the barcodes you would be more than likely looking at roughly 900 to 1000 characters. Exactly what you are experiencing.
I've watched that presentation a couple of times already, which is why we were expecting to get closer to the 2710 character limit. Due to the fact that we are using fax as the primary incoming channel for our barcodes, we've had to go with level 6 error correction - which obviously costs capacity. We also moved away from storing the full document XML based on the watching of that presentation also.