The ones I'd use aren't too large, about 1.5 inches cubed for the tap
connectors and I'm hoping the dimensions for the splicer is listed in
cm or otherwise those things are way way bigger than I expected.

As for the twist on wire nuts, have a read he

http://www.inspect-ny.com/aluminum.htm

Apparently they don't really improve the safety aspect of aluminum
wiring much. About half way down the page you'll find lots of info
about the Ideal 65 connectors that are approved for aluminum wiring.
They do offer a solution for using a different wire nut which I might
go with ... but I'm still debeating.

Thanks,
Harry