BEWARE: long post, the first half answers questions from your post, the second is my electronics recommendations.

I'd like to start by saying that a short part with lots of XY surface area is more likely to de-laminate than a tall part with a small XY cross section. Larger XY cross sections mean that each layer contracts more as it cools, this means larger forces between the layers and leads to de-lamination.

As for the arduino/RAMPS questions. I believe it has 5 stepper sockets on the board itself, one for each axis (but you can piggy back two motors off one driver, or assign two stepper drivers to one axis, or three if you really want to...?) and then up to two extruders. There are expansion boards you can plug into it for more stepper drivers. For your dual y setup with two limit switches, you'd need a dedicated driver for each Y axis motor unless you're ok with ramming the gantry against the y axis limit until both switches trigger. Nothing really wrong with this, the motors will just skip steps and as long as your printer is reasonably solid it wont hurt anything. It just makes a nasty noise that doesn't inspire confidence.
Side note, the two Y limit switches will do nothing to keep your build plate level, it could help keep the gantry square if you set them up so they trigger when it's square.

8 bit electronics (ramps/mega) will have no problems driving a cartesian machine at speeds well beyond what would shake it to pieces. It can have some trouble with nonlinear kinematics but even then it can manage unless you want relatively high print speeds.

The RAMPS/mega combo can use bed sensors, and it can run a heated bed though I suspect not one as big as you're looking to use. Could you please let us know how big of a print bed you plan to use?

By other sensors do you mean X/Y/Z max homing switches? RAMPS supports both a min and max endstop for each axis I believe. Even if it doesn't, it's not hard to add.


With all that out of the way, on to electronics suggestions.

Starting with the stepper drivers. You need 4 drivers for your movement axis, and are wanting up to three extruders. This means between 5 and 7 stepper drivers required. RAMPS can handle the single extruder case, but would need an expansion board for the extra extruders. I recommend against the ramps, spend a few extra bucks and get an all in one board with integrated drivers, it costs no more than 5 dollars extra but saves you a lot of wiring hassle and helps with stepper driver performance (important on a big machine where you'll be pushing the motors). Another reason I'd recommend against RAMPS for you specifically is that it's limited to 12 volts, this hinders the performance of the stepper motors which again is important for bigger machines. Your best 8 bit option (read: cheap) would be something like the RUMBA (no integrated drivers, so slightly poorer performance, but has 6 drivers which means you can use two extruders and double y axis) or a mks base or similar (integrated drivers will perform better, but this only has 5 drivers so it can only run one extruder with your setup). You can use stepper expanders with all of these boards to use an extra stepper or two.

As for the rest of it, min/max endstops are supported on just about every board, so are bed sensors. Most boards have pretty low powered (or even underpowered) mosfets for the heated bed, and they're usually gears for the standard 200x200 heated beds. To use a larger heat bed you'd probably want to use an external mosfet module. Again, I'd try and use 24 volts if I were you (i.e. not ramps), it boosts the performance of the stepper motors and heated bed/hotend. You could get your machine running with two extruders (or even three with an expander) using a RAMPS but you'd be making some pretty big compromises, so I highly don't recommend it.

Sorry for the long post, wanted to be thorough, let me know if you want a TL;DR