There's more to it than size alone. What you need to consider is when are you going to camp, what temperatures you anticipate camping out in, whether you are a warm sleeper or a cold sleeper, and your budget. Once you have those fixed and shared with us then you have a good chance of getting some meaningful answers.
Consider quilts too since, for the same weight and warmth, a quilt can be lighter and more compact than a full sleeping bag, as well as cheaper to buy (typically!). A quilt is basically the top 2/3 or 3/4 of a bag, so it goes over the top and down the sides, often with straps across the bottom to hold it in place. Very popular with ultra-light camping type people and there are now lots on the market. If I'm in a tent, I definitely now favour a quilt; same applies to being in a bivvy bag. The only time I'd prefer a full sleeping bag now is sleeping outside with no tent or bivvy.
Liners are good, but primarily for minimising the need to clean the bag. The most significant heat loss is to the ground, so the most important thing is a good insulating mat. You can prove this to yourself by lying on the ground and considering which side feels cooler. Within a minute or two, it's the side in contact with the ground. I bivvy up mountains a fair bit and I'd rather go without a sleeping bag/quilt than without a mat.
I've been thinking about this recently. I think a few considerations are:
How are you going to carry it?
Is space at a premium?
Temperature during use?
Temperature is the biggy, are you going mid summer and sleep warm anyway? In that case a 1-2 season synthetic can be bought cheap and they pack down quite small. If you're going for a 4 season plus bag to keep you warm in the middle of winter a synthetic would be huge and down may be you're only option. Then think about carrying it, if it's going in a large pannier bag, and will be on it's own, you could probably afford the space for a cheapo synthetic, if you're going ultralight then down again may be your only option. Another factor that I take in to consideration is cost. How much are you willing to spend?
I'll be adding a few bits to my blog later on this evening. I'll be posting up comparison pictures of a cheap crappy tesco synthetic microlite bag versus a cheap crappy Vangor wilderness 250 bag.
Both bags come in at the £20 mark, which is dirt cheap.
The Tesco bag has a comfort rating of 13*c, the Vango is 5*c.
The Vango weighs 1.3 kg, the Tescos bag is about half that. The Tescos bag also packs to about half as small. It'll make more sense once I've uploaded pics and weights etc.
I've done some research on liners and from what I can tell it seems they're primarily useful for preventing the need to clean the bag, and offer little to no insulation benefits. The Thermartex Bedding Blanket however, is supposed to work well. I think I'll be grabbing one to combine with the Tescos bag for spring / summer. During colder periods I'd just have to bite the bullet and lug the Vango bag around.
It's OK going for as small a pack size as possible but I tried sleeping in a mummy bag and got terrible claustrophobia, so much so that I was really panicked by the experience, I now use a large square cut bag, or two mummy bags zipped together to give me room to move!