Hello, there are a few things to consider when sizing tanks for your set. If you plan on doing a lot of cutting, the oxygen tank will likely empty out fairly quickly and require a lot of trips to the gas supplier. So if you live far from town you might want to go with a larger cylinder to cut down on the trips. Another thing to consider, if your tank usage requires carting them around a lot and to places where weight is an issue and portability is a bonus, you may need to make a slight trade off when you consider the size and go with smaller tanks. One of the more important issues regarding tank size has to do with the acetylene cylinder, depending on the tip sizes that you plan on using, some of the larger tips require a larger acetylene cylinder in order for the tip to operate safely. The tip size thing applies to welding tips, cutting tips, and heating tips, (commonly referred to as rose buds). Concerning the acetylene cylinder size: almost every tip that is available for use on oxy-acetylene torches has information regarding the tips fuel gas consumption rate, typically given in CFH (cubic feet per hour), all bottles of acetylene should be marked with their CF or cubic feet of contents, for an acetylene bottle to be used properly you should only draw off 1/7th of the total CF of the bottle per hour. An example: one of the large industrial sized acetylene bottles has 350 CF listed on the bottle, this means that the maximum amount of acetylene that can be drawn off of this bottle in an hours time would be 50 CF, a #1 cutting tip only requires 9.5 CFH to operate properly, there would be no problem supporting this combination, on the other hand there is a rose bud that could be hooked up to this unit and it's acetylene consumption rate is 90 CFH, if you used this tip on the unit it would light and operate, however, since it's draw-off rate exceeds what the bottle is capable of providing you would run the risk of drawing some of the acetone liquid that is part of the make-up of the bottle into the lines and regulator and ruining the components and also ruining the tip. The whole thing regarding draw-off rate has to do with acetylene gas to begin with, it can be a very dangerous substance if you don't respect it. Acetylene bottles have a steel shell that is filled with a porous material, sometimes called portland cement, newer bottles have a different type of synthetic filler, they also have liquid acetone in them. Simply put, acetylene in it's free state is very unstable at pressures over 15psi. and can react violently (explode!), thus when it is filled into the acetylene bottle the acetone and filler are there to combine with the gas to form a much more stable environment which allows it to be pressurized to the 250 psi. that is present in most bottles when they are full. Since it is combined with the acetone it requires a specific amount of time to essentially boil out of solution when you open the valve on the tank, that's where the 1/7th draw-off rate comes from. You've heard the safety spiel, now for one final comment. A number of equipment users out there are notorious for using pressure settings on their tanks of 7psi. on the acetylene and 40psi. on the oxygen, this isn't necessarily dangerous or anything but it can be very wasteful. When you set up your torch to cut look at the pressure requirements listed by the manufacturer for your particular tip size, by using these pressures you will be able to optimize the performance of the tip and not waste your gases. On the Victor tip chart you will notice that a #1 tip which is rated for 3/4" material will operate with 30/35 psi. of oxygen and 3/6 psi. of acetylene, particularly with the oxygen, using excessive pressures doesn't make the tip perform any better but it does waste a lot of unnecessary oxygen. Sorry for the lengthy story hope it helps. Regards, aevald