Is there a limit to rollover amount from solo 401K to IRA ?

I am self employed for 15+ years and have contributed to my own company's (LLC) in 401K and will also do for 2017.  I am 68 years old. As my consulting income fluctuates, I am thinking to contribute maximum for 2017 (50K+) and minimize my taxes for 2017.  In 2018 I may have to withdraw money from this account if I don't have income in 2018.  I am told that they will hold 20% tax before 401K distribution.  What if I rollover entire 401K in 2018 to IRA and then withdraw without having to withhold 20% tax which may be much more than I would pay in 2018 for zero income.  Please advise and pardon my ignorance.