If your not sure if you will need a sleeping bag for 12-25C try it, go out in your back yard in similer weather with similer clothing and sleep overnight.

I have a one person tent that was given to me by a casual hiker that said he gets a little cluasterphobic(feels big to me), its a Wenzel but I don't know the model, it's prism shaped like a pup tent. It worked excellent for stealth camping around the western usa, it sets up and packs as quickly or quicker than a dome tent and has just enough room for paniers sitting or changing.
Packed its just under 15"(38cm)long x 5"(12cm)diameter, it weighs one kilo(2.2#).
Set up the front triangle is 45"(114cm)Widex35"(89cm)Tall, the foot triangle is 34"W(86cm)x23"T(59cm), 78"(2.0M)long.

Someone mentioned a sleeping pad and bag I haven't tried it but I was just thinking you could stich the top half of a sleeping bag to a pad(not the air filled kind obviously) and use a liner. the whole thing would roll up not much bigger than the pad alone and woould save a half sleeping bags weight.
Down stuffed anything cannot get wet it will mat flat and loose all insulating value, it will return to normal once dry but that could take a while; dry weight to dry weight down is a bit lighter and packs smaller for a given temperature rating then synthetic fill. The synthetics don't mat when wet and keep almost all their warmth.