The websites for the sleeping bag manufacturers should include the dimensions of the bag. Remember that "shoulder" is measured over your arms (it's NOT chest measurement) and should include a puffy jacket when measuring, with enough ease so neither the sleeping bag insulation nor the jacket insulation is compressed in any way. (The jacket is your safety margin should the temp dip below what's expected.) We females also have to worry about hip measurement, not always given in sleeping bag dimensions. Unfortunately, the lightest weight bags tend to be the skinniest!

Temperature ratings of sleeping bags can be a bit dubious. If the bag is not EN13537 rated, the rating--at least for inexpensive bags--may be a purely fictitious one made up by the manufacturer's marketing department. The EN13537 ratings are not exact science (everyone's metabolism is different) but at least involve laboratory testing. Again, check the manufacturer's website. The ratings for the really high end bags, such Western Mountaineering and Feathered Friends, are more trustworthy.

Synthetic insulation doesn't bounce back from being compressed as does high quality down, so even a high quality synthetic bag will lose some of its loft in a few years. That's probably why the new bag seems warmer!