Wines come in all size bottles. The most standard wine bottle size is 750 ml. The 'half-bottle' at 375 ml is used sparingly for table wines, but is a common size for dessert wines. (The wine makers may be concerned that after a couple 750 ml bottles over dinner, it is good to finish with some restraint, so a smaller bottle is in order!) The smallest size is the 'split' at 187.5 ml, and is known as a single serving size. This is the size you are usually handed or asked to buy in economy class on an airline.
Then there is the impressive 1.5 liter 'magnums.' The sheer mass to these bottles make them an impressive sight! For larger groups, they can prove useful to make sure everyone gets a taste. But after that, the even larger bottle size starts to become almost ridiculous. The 3 liter 'double magnum' is more suitable for large parties where you want everyone to drink the same thing with limited choice. This may serve well at a BBQ or a wedding, for example. But after that, I believe the size of the bottle is strictly for show. I mean how does one lift and pour the bottle, and how does one decant the bottle? You would require a bathtub for a 12 liter bottle!
Except for dessert wines, I almost always drink only 750 ml bottles. They pour and decant nicely and without much drama. The standard decanter is designed for this size bottle unless mentioned otherwise. The main reason I like to drink 750 ml bottles is that when having a dinner or gathering of four to eight people, then I can open two to four different bottles of different wines. This works really well in terms of matching wines to different courses of a meal, and also for comparing and sampling many different wines. In general, I find it is always more pleasurable to share a couple of different bottles instead of focusing on only one wine in an evening. Larger bottles such as magnums (or even larger) limit the variety for smaller groups.
I have about 15 magnums of some nice wines I plan to use for larger celebrations and when bringing some special groups of people together. I also bought (as a really good deal for the volume!) 24 double magnums. The double magnums represent some of Australia's best wine from the best vintages. Yet, every time I think about pulling one out, I usually go with other choices (such as several bottles of a comparable wine) because (1) the are easier to handle, (2) I have more choice, and (3) the larger bottles are unlikely to be ready to drink yet.
When wine ages in a bottle, the air in the head space (called ullage) mixes over time with the wine in the bottle to mature and enhance it. Describing why and how this happens could take several posts on their own. The important thing to know is that for larger bottles, the ratio of the volume of air in the head space to the volume of wine is drastically reduced (since each bottle is filled to the top). Therefore, it takes a lot longer time for the wine to mature and reach optimal drinking in a larger bottle.
When a wine taster / reviewer projects the time range in which to
optimally drink the wine, he/she is estimating that based on the wine
being cellared in a 750 ml bottle. The general rule of thumb (for good wines that enhance through cellaring) is that for every doubling in bottle size (from 750 ml) you should cellar the bottle for an additional three to five years. Therefore a double magnum could take an additional six to ten years in the cellar before it is ready to drink.
Unfortunately, there is also the risk involved that the cork or the storage conditions are sub-optimal and statistically, you may not drink the bottle at the right time for that bottle. There is a much greater chance with larger bottles that you will either drink the wine too early or too late.
Two years ago, I opened a 750 ml bottle of the 1994 Wynn's John Riddoch (a Coonawarra Cabernet Sauvignon). This is a stunning bottle of wine and drank beautifully. We were able to share it among friends as part of a wine tasting. Then several weeks later, I took my team out to celebrate the successful end of a major project, and over lunch and after a few earlier wines, decided to buy a magnum of the same 1994 Wynn's John Riddoch, expecting it would be comparable. However, it was still too tight and even after much decanting, could not compare with the same wine stored in the 750 ml bottle. The magnum required at least five more years to be as soft and mature as the wine from the 750 ml bottle was at that point in time.
This was an expensive mistake, but I learned a lot that day. I now take bottle size seriously in laying down the wine for the requisite number of additional years before I consider opening it.
In general, I avoid or limit the number of large bottles I have, but if you do have some, make sure to age them for a longer time. The general rule of thumb should be to add three to five years of cellaring every time you double up the bottle size from the standard 750 ml size.