What is a storage need for MySQL index on Varchar(100) in InnoDB?
I have discussed having large PRIMARY KEYs for InnoDB before : What storage engine should I use for this MySQL table?. The effect would be bloated keys experiencing linear growth of all Secondary Indexes because the PRIMARY KEY reference from a Secondary Key Index Entry would also start getting bloated as well.

Looking at your question, you yourself are saying you will be expanding an email address from 20 to 64 characters. You will be bloating every non-unique index for the table in question that has an email address column. If the email address is itself the primary key, then all indexes for the table in question whether an index has an email address column or not. There are other viewpoints discussed in Mysql int vs varchar as primary key (InnoDB Storage Engine?

It is bad enough that a CHAR field is fast to read than a VARCHAR field but at the expense of having larger indexes (See my post What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?). Doing this  will certainly introduce configuration challenges including


Having a Large Enough InnoDB Buffer Pool (innodb_buffer_pool_size)
Having Large Enough Transaction Logs (innodb_log_size_size)
Having Sufficient Read and Write IO Threads (innodb_read_io_threads,innodb_write_io_threads)
Having a Large Enough Log buffer (innodb_log_buffer_size)


Even if email address is not indexed, data pages for that InnoDB will still have bloating and possibly fragmentation nonetheless. Since you are hashing, I can presume you must be indexing it.