What is the typical size of an accounting department for a $5 - $10 million per year company?

Depends! On number of transactions required to be posted everyday, and how much credit is extended. Also, if a bank/lender is involved, then there is a lot of reporting. If an audit needs to get done, then a plan to factor that level of skill is required. Typically, a Senior Accountant/Controller, a Bookkeeper/accountant. If collections are required, an A/R and possibly an A/P person. That's 4 persons. If Customer service i a different dept, maybe the A/R can be combined there...and so on. All other services can be outsourced as required from time to time.

Rejeev is right. The number of transactions is key to determine staffing levels. But I've seen a number of businesses that size do fine with one very good office manager. Also, consider adding a part-time CFO to add real value, especially when professional money (banks, institutional lenders, equity investors, venture capital and other pros) is involved.

Again agreeing with the "depends" comment;
I would posit 2: If you can share responsibilities between a few people with other day jobs (ref: office manager, in small companies there are many) you can keep a good lid on it. 2 is the effective minimum as you'll want to split posting from approvals. As Rajeev indicates heavy AR/AP can add to this, for example if you've got a few hundred mid-sized customers with payment terms. Consumer or large enterprise customers tend not to consume resources as greedily.

I will continue the "it depends" theme, but with three examples from recent experiences.

$4mm revenues -- 10 total staff -- uses one F/T bookkeeper (limited skill sets/experience) and a P/T consulting CPA for monthly review, state reports, etc. and me visiting once a quarter for high level oversight/bank relations/planning with CEO.

$20mm revenues -- 41 total staff -- high invoice count and large customer base -- one A/R and collections person, one A/P, payroll and misc accounting person, and a 50% P/T financial overseer.

$5mm revenues -- 35 total staff -- uses one F/T bookkeeper/accountant for everything (not that skilled and it shows. Quite a mess 'cause they don't want to pay for more.)