Correction: After this answer was fully developed and consistent with what was available, a new "hint" was added that invalidated most of the answer. With the new information, the answer is more likely to be
with some of the same reasoning below still applicable.
However, after seeing that significant late change, I am no longer investing time into this one or likely to participate in other puzzles by this OP.
and the letter is
C, for Clear
I come in many shapes and sizes.
Calculators do come in many shapes and sizes.
Some forms have even won some prizes.
Some forms/designs are very impressive and have won prizes.
But most of us look much the same.
The basic set of buttons and a display is pretty common.
Speed you can gain with some game.
Using a calculator can make you faster at arithmetic.
But what is my name?
To use us you must have some tricks.
You do have to know how to use it, and it helps to know arithmetic rules like commutative and associative properties.
For there are us two times six.
12 buttons on the number pad
You most likely haven't seen us in a while.
We are probably sitting in some pile.
Think of us, and smile :)
Calculators have fallen into disuse with more powerful smartphones and computers becoming more common, and the ones people used to use are probably sitting in a pile of "stuff" or even a landfill.
Hint line 1:
I am something you might use everyday,
A device to assist with basic mathematical calculations might be something you use everyday.
Hint line 2 (Did you know spoiler tags don't work if preceded by a blank line?):
Or maybe not, if your hair is gray.
People who grew up in a time when calculators were not assumed to be generally available when needed may have just memorized the arithmetic.
Hint lines 3-4:
My ancestors were great,
They had significant weight.
Old computing devices were large and heavy.
Hint line 5:
Our functions are plenty,
Depends what type of calculator you have, but they do tend to have various functions.
Hint line 6:
Together, we are over twenty.
Basic arithmetic functions can be combined together in >20 ways. Also, if you count the function buttons, there are >20 buttons on a basic calculator.
Hint line 7:
With some numbers and a letter,
The letter is C, for Clear.
Hint line 8:
For us to be gone, maybe its for the better.
Calculators have generally fallen into disuse in favor of more powerful devices, which may be for the better (depending on your perspective).