MySQL `innodb_log_file_size`: how big is too big?
You have to find the right tradeoff between recovery time in the event of a crash and how much write throughput your database will be expected to support. 

Percona has a great post discussing a similar scenario to yours, with a 64GB buffer pool, and about 2/5ths of the way into the article you can see a nice comparison he does between a 2GB and 15GB logfile. 

15GB allows innodb enough log space to 'smoothly' flush pages out of the log, whereas with 2GB there is more pressure to keep the log from filling up, which ultimately hurts performance as pages have to be evicted. In the benchmarks they cite, performance is 20% lower as a result. 

TL;DR you can't really go wrong with a larger log file, esp. if you have a slave to failover to in the event of a crash, but you can certainly hurt performance with one that is too small.