As the subject says, what's the accepted size for a mailbox these days? I am trying to decide how much space to give to my users on a new mail server I am setting up...

couple of hundred megs maybe? , i guess it depends if your using pop or imap

emails don't take up too much space unless there is a lot of attachments - Kar's guess of a few hundred megabytes sounds about right. It could hold thousands of emails on that space assuming attachment count is low.

1gig is a great size, what I initially assign to new customer. If they request more I have no problem giving it!

I get hundreds of emails per day, and have over 10,000 saved emails and I am only using a few GB's w/ attachments.A gb or two is usually enough for most people.

I see no reason in needing over a gig. POP that **** and burn it if you need to save it.

1 Gb should be Ok, most people will never reach it, MOST...

A client of mine get's more than a GB of mails a day, he is into printing business, so he get heavy attachments. But that is a rare case.

As Jacob mentioned a Gb or two is good enough. If you want it to look more lucrative that will be a different story.

1GB is usually ample for most people... I have over 50k emails saved in my inbox at present and Exchange lists it at just over 1200MB. Usually when I hit 70k I start cleaning out :)

I think 50 MB is a decent size .

100-150 MB I assume that is more then enough

This depends on if you use POP/SMTP to download all your emails in your mail client without keeping a copy on your server. If so there is nothing to keep in the web-mail and you will not need much space for your email accounts.

I am sure you are not talking about Email Hosting ...or are you ?

If you are not talking about email hosting, then several hosting panel support where summary Disk space usage = Email+ database+ webFTP+ logs

Benefit of this system is, even if you allow a default size of 50MB per email id, customer can increase it up to package disk space limit without no additional charges.

but usage is really depends on the need of the customers. If he use only IMAP then he will be needing a little more space then he is using pop for his emails.

Start off by giving them just 1.5GB then if they need more I'd bet they'll contact you.