Yes, you can do partial conversions to keep the amount of tax you owe for any given year under control.
There is no minimum or maximum conversion amount in IRS regulations. (Your IRA account custodian might have transaction minimums.)
If you do a direct rollover (direct transfer of funds from one trustee to the other trustee) you can do that in any amount and as often as you like.
An indirect rollover is where the IRA custodian sends you a check or deposits money in your account, and you send it to the new IRA custodian. You must deposit the money with the new account within 60 days and you can only do one indirect rollover per year. Indirect rollovers are also subject to mandatory withholding, which can cause problems. For example, if you want to convert $10,000 by indirect rollover, the custodian will withhold 20% ($2000) and send you $8000. If you deposit $8000 in the new account, that counts as an $8000 rollover and a $2000 withdrawal. If you want to roll over the entire $10,000, you will have to find another $2000 from other funds. (The withheld amount will be applied to your tax return, but your actual tax owed may be more than 20%, depending on your other income. Any amount treated as a withdrawal will be subject to an additional 10% penalty for early withdrawal unless you are over age 59-1/2.)
Finally, remember that since Roth conversions are taxable, you may owe estimated tax payments to avoid a penalty. The tax system is pay-as-you-go, and if you have a large lump sum of income, you may be required to make an estimated tax payment before the end of the tax quarter, or else you may pay an underpayment penalty even if you pay in full at tax time. To avoid an underpayment penalty, the amount you owe at tax time must meet one of these tests.
1. You owe less than $1000.
2. Your payments via withholding plus estimated payments is at least 100% of last year's tax liability.
3. Your payments via withholding and estimated payments is at least 90% of the current year's tax liability.
