wire primary/secondary switch

Yes, that would work as long as the diode has enough current capacity.  If you are only running to small LEDs then it won't be a problem - a 1A diode will work.

If you plan to pull a decent amount of current through the switch you will need to keep the same conept but use relays.  You can diode isolate the relays so that it works the same way you described above, but since you are only switching a relay you can keep the small 1A diode.

Physically, how big is that diode?

Yes, radio shack sells a 6 amp diode.  It is much smaller than the one you posted dimensions for. 

< color=#0052b4>

< color=#0052b4> < color=#0052b4> Print < color=#0052b4> < color=#0052b4> < color=#0052b4> Email a friend < color=#0052b4>< color=#0052b4> < color=#0052b4> < color=#cc0000 >6A, 50V Rectifier Diodes (4-Pack) < color=#2a2a2a>< class=ourprice>$2.49 < color=#2a2a2a > Model: 276-1661 Catalog #: 276-1661 Online: In stock Usually ships in 1 - 2 business days < ="return verifyFields('no',.order);" method=post name=order =/cartHandler/index.jsp>< value=skuAddToCart = name=> < = name=wlName> < value=2032058.2032230.2032269 = name=cp> < = name=showMy> < value=1 = name=qty_0>

Disregard the text about relays on this picture.  It was intended for using one switch for parking lights and headlights.