What is the size of currently-existing tmp tables in innodb?

Question

This is yet another "how to add a column to a gigantic table" question. The table on disk takes 2.5GB, and I have around 10GB of ram I can use. I have a feeling I can crank up the tmptablesize option so my add column tmptable resides completely in memory so disk i/o isn't a bottleneck. How can I get a sense of what value I should set tmptablesize to?

I have a second vm provisioned for a dry run. I'm running my alter table command and it is chugging along, writing to a temp table. How can I tell how large this temp table is, exactly?

edit for comments

mysql --version
mysql  Ver 15.1 Distrib 5.5.30-MariaDB, for debian-linux-gnu (x86_64) using readline 5.1

The table has 5.5 million rows: 10 decimal columns, 17 integer columns, and 2 datetime columns. No foreign keys. 12 of the integer columns have indexes.

The command:

alter table anonymized_table_name ADD COLUMN `anonymized_column_name` VARCHAR(100) NULL;