My 2016 Classic is at the dealer getting a few items fixed, so I cant give you exact dimensions. But if a good "guestimate" is Ok:
1. The drawer is about 22in wide, about 20in long and about 6in deep. There are wedge-shaped supports inside the drawer at front center and back center - these wedges somewhat reduce the available height by about 3 inches at the immediate front and back of the drawer. I'd say you could fit a 20x14x4 inch package in that drawer.
2. I happened to take a quick look under the front dinette seat weeks ago (when an upholstered panel cover came loose). I saw what I assume is the inverter attached to the street side wall. The electrical diagram in the manual confirms this, and says the transfer switch is there too. Wild guess: the inverter equipment "area" seemed to be about 1 ft wide, 9 inches tall, and about 4 inches deep.
3. The converter is located curbside, at floor level, behind a snap-on finished wood cover located at the left-side base of the wardrobe closet. Behind the wood cover is the usual hinged metal cover for the converter, AC breakers and DC fuses. The visible external area is about 1ft tall and 1ft wide. The literature for the Parallax 5355 unit says it is about 12H x 15W x 14D (in)
4. The button that turns the inverter on/off is located on the curbside wall between the left edge of the stovetop and the right side of the galley window, a few inches above the countertop. As I mentioned in #2, the electrical diagram says the inverter "transfer switch" is located with the inverter under the dinette seat, but I'm unsure if you are referring to that, or to the on/off button.
Hope that helps. If not, perhaps someone else with more precise information can step in.