How large is a googolplexian?

To properly explain a "googolplexian" you should first start by understanding a "googol" and a "googolplex".

First, a GOOGOL is a 1 with 100 zero's after it (10100):
10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000

Next, a GOOGOLPLEX is a 1 with a GOOGOL of zero's after it (10googol).

Finally a GOOGOLPLEXIAN is a 1 with a GOOGOLPLEX of zero's after it (10googolplex).

This web site best demonstrates just how many zero's are in a Googolplexian:
http://www.googolplexian.com/