Does anybody have any thoughts on using a long winter bag when a regular size is what you normally use? I know some people store items in their bags in winter, just wondering if it's considered 'mandatory' or not.

This is obviously going to depend on what you want to keep in it. Last winter I decided to drop the tent and just try using a bivy. It worked out fine and I will probably rarely use my tent for CA winter camping again. The first thing I did notice though is that I need a longer bivy to fit my boots or liners and what ever else down there that I didn't want to leave out. Think about what stuff you need to keep warm or dry out while you sleep and if it fits in the regular length with you, then you're set. If not, then size up. In my case, I think the regular bag with the long bivy will be fine. We'll see how this winter goes.

I'd be interested to know what sort of bivvy bag you're using, because I'm thinking in a similar direction. My trips are mostly in the Bavarian & Austrian Alp, where tents are (a) illegal in most national parks, (b) not really practical above the treeline. So bivvying seems like the way forward.

I'm looking at the moment at ex German army goretex bivvy bags on ebay. They are, at around a kilogram, admittedly big & heavy compared to a lightweight bivvy bag - but light compared to a tent - and cheap at around 50 to 70 Euros, compared to well over 200 for other goretex or eVent bags.

You're weighing the ability to bring things like boot liners into your bag with the extra air you'll have to heat up.

So,

How big are you versus your bag (height and girth)?
How cold are you going (the colder it is the more desirable keeping water bottles unfrozen becomes, but the temp stakes are also higher)?
How much will you be pushing your bags temp rating?
Do you sleep cold or warm?

And so forth.
For myself, I'm 5'11" and find normal 6' bags fine, but I'm also rather thin and can but bottles and electronics along my legs and it doesn't bother me.

I'm 5'10" and 145ish lbs, so I've always found 6' bags to be fine. I don't plan on any extended winter trips, just 1-2 night trips mainly here in the Tahoe basin where the night temps will generally be 0 to 10 degrees (I plan on going out when I know it won't be particulary nasty).

I plan on using a 10 degree bag with down pants and down jacket to supplement when needed, inside a tent or pyramid tarp either solo, with my dog, or with my wife and dog. I imagine I would want to keep boot liners and maybe some water bottles inside the bag, but not really anything else.

This will be my first winter camping season, so it's strange to be asking noob questions again in regard to backpacking.

At your weight especially, I'd go with a "normal" length bag. The only thing I'd change with my WM bags is to make them a bit skinnier.

For single digits I keep water, gloves and socks, and especially all electronics inside the bag at night. Cold kills batteries.

"You're weighing the ability to bring things like boot liners into your bag."

This is very true. I don't have winter boots with removable liners. So I don't really have much bulky stuff to store in my bag.

The -20F Montbell bag has a great feature. If you buy the long, you have the option of storing stuff in the bottom, or turning the extra length back in on itself. This creates a super warm, comfortable pillow for your feet.

Alan, the bivy I'm using is homemade, dry ducks on top and was black 3 mil plastic on the bottom, but that is changing to tyvek right now. The plastic was getting a lot of pin holes in it.

As for the reg or long decision, I agree that based on your size stick with the reg, and put stuff around and under your legs

Both of my winter bags are long (summer WM Megalite is regular length).

I need a long winter bag for keeping water, batteries and breakfast energy bars warm. Best to eat an energy bar before getting out of the sack to bring up your blood sugar a tad & keep you warmer while cooking breakfast and breaking camp.