My ideal saddle wedge is just large enough to fit the essentials. It is lightweight, easy to open and close, and attaches securely. I also like it when the bag has a loop in the back for a tail light attachment. There are lots of options for this type of bag. Here are the three I use on my bikes.
Jandd Hurricane Mini Mountain Wedge for the past year. As far as this style of bag goes it is on the larger size, but still very compact and light. Dimensions are 6x10x20cm, volume is 2L and weight is 104g. Made of vinyl-lined cordura, this bag is water and abrasion-resistant. Price is $26.95.
several versions and lots of colours, as well as in leather, suede and waxed cotton (though the latter will increase the weight of the bag and reduce its waterproofness).
Inertia Designs as a supplement to my handlebar bag. This is a smaller style of wedge, with a pyramid-like shape to reduce its width. Dimensions (measured at widest parts) are 10x10x14cm, volume is .7L, weight is 70g. Made cordura lined with truck tarp, the bag is water and abrasion-resistant. The bag is US-made. Lots of colours available. Price for the version I have (embroidered with Harris Cyclery's logo) is $21.95.
Soma Noe Road Wedge, which I typically affix to demo bikes that I test ride or have on loan for review. Soma does make larger saddle wedges, but the "narrow aero" Noe is truly minuscule and will fit just the essentials. Dimensions (measured at widest parts) are 15x9x4cm. Volume and weight are not stated. Made out of a hemp and cotton blend fabric with a waterproof coating, it is water resistant. Available in black and khaki, as well as in an all-reflective fabric. Price for the standard version is $12.99.
If you browse through the saddle wedges offered by different companies, you will notice a pattern to their shapes, sizes, closure systems and other features. Some are flat, others pyramid-like. Some use buckle attachments, others velcro. A good way to determine which works best for you is to visit a bike shop that stocks different models and buy the bag there. Also talk to riders who have used specific bags for a while and in different weather conditions. After destroying the Fizik wedge I initially had on my roadbike, I was steered toward the Jandd by a local rider, which has since endured heavy use and bad weather without a mark on it.
When you don't want to carry a lot on your bike, the saddle wedge is a great little bag that keeps things simple and functional.