MySQL database size estimation

I have an application database with a table for users (1kbyte of data per user based on counting fields * typelength), and about 100 things of the same size belonging to a user (0.5 kbyte per thing), and it is in a "user" table and a "thing" table.

That would seem to lead to about 51kbytes of data per user. However, I have heard that for MySQL, I should double it to cover index tables, which would get me to 102kbytes/user Is that true? Are there any other data expansion factors to consider for MySQL, or is 102 kbytes a good estimate?

Besides the indexing factor (which I think is 2) and the storage efficiency (which I also think is 2), are there any other multipliers for data storage in MySQL?

Short answer
Size increase 2-3x over MyISAM is common, 4x is rare.

Everything about the InnODB engine:
http://dev.mysql.com/doc/refman/5.1/en/innodb-storage-engine.html

InnoDB, MyISAM and disk space:
http://mysqlha.blogspot.com/2009/01/innodb-myisam-and-disk-space_16.html

MySQL engines space usage comparison:
Part1: http://marksverbiage.blogspot.com/2008/02/mysql-engines-and-space-usage.html
Part2: http://marksverbiage.blogspot.com/2008/04/mysql-engines-space-usage-comparison.html

Here's the physical row structure:
http://dev.mysql.com/doc/refman/5.0/en/innodb-physical-record.html

There are a lot of variables and issues:

Indexes, remember InnoDB includes the PK in every secondary index.
Are you packing keys (slow)?
Is the table redundant?
Don't forget about the logs (binary log, slow query log, error log ....)
Are rows declared as nullable, if so add an extra byte per nullable column per row.
What charset are you using?