Hey Joe,
Having a large number of rental properties and installment sales can certainly make your tax return voluminous, but as long as you're maintaining accurate records, filing everything correctly, and reporting all income, the size of your return itself is not necessarily an audit trigger. The IRS typically selects returns for audit based on unusual or suspicious patterns, not the sheer length of a return. In your case, with a history of passing audits and having a CPA who is experienced in IRS representation, you're in a strong position should an audit occur.
Regarding audit statistics, the IRS generally audits fewer than 0.5% of individual returns, with even lower rates for those earning under $1 million annually. In fact, those with incomes under $1 million have about a 0.2% chance of being audited, with the rate increasing slightly as income rises. The key factors that may raise audit risk are discrepancies in reported income, excessive deductions that appear out of line with your income level, or unusually complex transactions. Since your CPA is advising that your current structure is sound and not worth changing, it seems you're well prepared, especially with your past audit experience.