In addition to Charles' comments...
Note that while a Double can have in excessive 300 digits of "precision",
only the first 16 or so digits are "accurate". This can lead to a very long
discussion, but while you might be able to get a very long number, it is
really only an approximation, of which the first 16 digits or so are
accurate.
Note how they described the limits:
1.79769313486231570E+308 for positive values
They use that notation for a reason. Only the digits listed could be
considered close to accurate, the other 291 digits it can "display" are only
an approximation.

You started by saying that you need a very large positive Integer. Well, if
that is what you need, then you should stick with Integers. Going back and
forth can cause you to lose accuracy.

Gerald

Thanks for putting it into perspective.  The answer is I don't really
know how big a number I will need.  This is a counter that counts up but
I really don't know how fast as it will be based on usage.  Being
careful makes me try for a type that'll hold a number it'll never hit or
perhaps I can reset it to 0 at some value but I'm not sure yet.  Frankly
I'll have to do some thinking on that.  double, decimal are I'm sure
large enough I wouldn't have to worry about reseting it.  Frankly a
integer might be ok but I'd probably have to put in a reset just in case.

anyway thinking about all this makes me wonder about how they all fit
and which ones I'll probably use in the future.  mostly I use decimal
and integer now.

The important thing is, for each range, to look at the signs of the
exponents.

The sign of the exponent indicates the direction in which to move the
decimal point.

+ means move the decimal to the right

- means move the decimal point to the left

To simplfy it for demonstration purposes let's take a floating point type
that allows the ranges:

   -1.79E+3 to -4.94E-3
   and
   4.94E-3 to 1.79E+3

-1.79E+3 equates to -1790
-4.94E-3 equates to -0.00494
4.94E-3 equates to 0.00494
1.79E+3 equates to 1790

Remembering that not every number can be represented exactly in floating
point, you can see that we have ranges of:

   -1790 through -0.00494 negative values
   and
   0.00494 through 1790 negative values

1790 is the upper limit, -1790 is the lower limit and the closest
representations of 0 that we can achieve are 0.00494 and -0.00494.

Now, extrapolate that example up to the Double type and you can see that the
ranges are:

   -1.79769313486231570E+308 through -4.94065645841246544E-324 for negative
values
   and
   4.94065645841246544E-324 through 1.79769313486231570E+308 for positive
values

This can be rewritten as:

   -179769313486231570[...300 zeroes...]00000000
through
   -0.000000[...310 zeroes...]000494065645841246544
for negative values
and
   0.000000[...310 zeroes...]000494065645841246544
through
   179769313486231570[...300 zeroes...]00000000
for positive values

This shows that 0 cannot be represented exactly as a Double and the closest
representations of 0 that can be achieved are actually:

   -0.000000[...310 zeroes...]000494065645841246544
   and
   0.000000[...310 zeroes...]000494065645841246544

Now, when it comes to dealing with the Decimal type, it holds a signed
128-bit (16-byte) value representing a 96-bit (12-byte) integer number
scaled by a variable power of 10.

The scaling factor specifies the number of digits to the right of the
decimal point and ranges from 0 through 28.

With a scale of 0 (no decimal places), the largest possible value is
+/-79,228,162,514,264,337,593,543,950,335
(+/-7.9228162514264337593543950335E+28).

With 28 decimal places, the largest value is
+/-7.9228162514264337593543950335, and the smallest nonzero value is
+/-0.0000000000000000000000000001 (+/-1E-28).

As you can see, for each decimal place that you have, you lose one integral
place and vice-versa.

With no decimal places you can have the equivalent of a 29 digit number with
a range of +79,228,162,514,264,337,593,543,950,335
to -79,228,162,514,264,337,593,543,950,335.

This is 10 digits wider than an Int64 (Long) and the maximum value is
approximately 8.5 bollions times larger.