What is largest value (number) that I can store in an Excel VBA variable?
The existing answer is correct in that you need to use the Double data type. That solves the problem, now I'll bring in some technical details.
The largest number that the Double data type can store (also the biggest number Excel can deal with) is 1.79769313486231570 • 10308. The biggest number that you can put in a cell (without a formula), however, is 9.99999999999999 • 10307. Note that you'll lose a good deal of precision when you're working with numbers that huge - adding a comparatively tiny number to things of that magnitude has a good chance of doing nothing. Floating point can get a little weird.
The maximum size of the Long data type is a drop in the ocean compared to Double's range; a signed 64-bit integer can only go up to 9,223,372,036,854,775,807. At least you're guaranteed to lose nothing to rounding there, as long as you're dealing with only whole numbers.
Sources: Excel specifications and limits, data type summary