“So the whole book of Mormon is supposed to have been translated from these gold tablets?”
Yes.
“Just how big were these things and how much did they weigh?”
If I recall correctly, their weight was estimated as being around 50 pounds. I can’t recall any description of ‘page’ size or the overall thickness of it.
“And how could anybody have just *lost* them?”
They were not lost, the angel Moroni took them back for safe keeping after Joseph was done.
Others have 'estimated' a higher figure.