How Big is an Excel worksheet?

Excel 2007, 2010, 2013 for Windows and Mac Excel 2011

Each worksheet has 16,384 columns and 1,048,576 rows.
16,384 × 1,048,576 = 17,179,869,184 cells

Note: However, it doesn't mean that you can happily place 17 billions numbers/formulas into a worksheet! It depends on the performance and vailability of physical memory of your computer. If resources are limited, Excel may encounter unpredictable instability while working on a large file.

Excel 2003 and below (Mac Excel 2008 and below):

256 columns and 65,536 rows

New Size = 1024 times of Old Size

If you tried to calculate this Excel formula:
=16384 * 1048576 / 256 / 65536
You'll find that the "new" Excel worksheet dimensions is actually 1,024 times of the "old" size! (This is obviously a computing consideration of Microsoft to do this.)

If you're a macro (VBA) developer, there is something you need to bare in mind:

In the "old" Excel dimensions, you can happily use an Integer variable to refer to the number of columns (e.g. in For-Next loops). However, such VBA macros may return an "Overflow" error. You should use either the Long or Single variable type to describe number of columns (and rows).