Since Bacchante was brand new, she took longer. Valiant's conversion should take about three months, total, then some time for fitting out (rigging, stores, gold leaf, etc.).
That's a little difficult to say, because in this era, size was limited by how much stress a wood hull could take, and that limited the length and width. The biggest frigates were ships like the USS Constitution and the French (then British) Pomone. They carried 24 pound cannon on their gun deck, as compared to other frigates, which carried 18 or 12 pounders. Ships like the Indefatigable and the Valiant are probably more powerful than the Constitution and Pomone by virtue of their larger scantlings (they were built originally to be ships of the line) and their larger secondary armament (they had 42 pound carronades on the upper deck); but in exchange for that, they weren't as fast and maneuverable, or as weatherly. So we would have to class Valiant as being one of the largest frigates out there. If I were to draw a more modern parallel, I'd say that Valiant and Indefatigable were analagous to the German Graf Spee pocket battleships.
You didn't ask, but this is the ship that was, IMHO, the best British frigate built: HMS Endymion