Amperage required for new sub panel
If you're spending the money to add a panel anyway, you might as well reduce the changes of having to expand it later.    Unless you're installing the second panel really far from the main, you're likely not going to spend much more to put in the 100 amp panel over a 60 amp panel.  Oversizing the panel will not hurt anything but your wallet, so why not do it?  

The installation will be exactly the same either way, the only differences will be the size of the breaker in the main panel, the main breaker in the new panel, and the size of the conductors between the panels.

You can read this answer for more detail, but likely you'll need four 3 AWG copper conductors to feed a 100 ampere panel.  Of course the size will change based on the length of the run, and other factors. So make sure you verify the size once you know exactly how long the wires will be, and how they'll be run. 

Did a quick search for wire prices, and it looks like it's about $0.20 per foot difference between a 6 AWG stranded copper wire and a 4 AWG stranded copper wire. Then another $0.20 per foot difference between 4 AWG stranded copper wire and 3 AWG stranded copper wire.  So you'd be looking at an $0.80 per foot difference between 6 and 4 AWG feeders, and an $0.80 per foot difference between 4 and 3 AWG feeders.