Breaker Feeding a Transfer Switch
That is the right way to do it! A transfer switch to an ordinary subpanel. Remember to separate neutral and ground at all points.   

You would use a 100A breaker.  The 100A is at 240V, so you could put 80A of 240V heaters onto that breaker/sub.   (because you must derate heater loads by 20%). 

At 120V it is good for both legs. That is, with 120V loads, you can put 100A  onto pole L1, and another 100A onto pole L2.  (assuming they are loads that do not need derate). 

Make sure to get a Rather Large panel. You will find your generator can support a lot more circuits than you might think.  Also, plan for a whole space per circuit: use of "twin/duplex" double-stuff breakers cannot be relied on, because most circuits will in the future need AFCI, GFCI or both, and those specialty breakers are not made in double-stuff.  

The subpanel can have any amp rating 100A or larger - 225A is fine.   That panel does not need a main breaker, and if it has one, the size doesn't matter.  The breaker in the main panel protects it, the wire to it, and the transfer switch. 



If you don't need an automatic transfer switch, some panel makes (Siemens) allow very cheap transfer switches - using a factory kit that straps two backfeed breakers together, and interlocks them so they can't both be on at once.  For Code compliance reasons, (well for safety reasons), 


the breakers must be physically bolted/strapped down so they can't come out like a normal breaker. 
The interlock (that keeps both breakers turning on at once) should not be disabled if you remove the panel cover.  That precludes many aftermarket kits that bolt to the panel cover.