All Lee presses including the hand press are strong enough to size the largest of magnum cases. As with presses of all brands, it is important to ensure that the case is properly lubricated prior to any attempt at sizing.
Proper lubrication requires the use of a good quality lubricant such as that made by Lee. We purchased the finest automotive deep draw lubricant and had it custom packaged exclusively for us. It is pleasant to handle and you can thin it with water. Non-sticky, nonallergenic and very little is needed;it goes a long way.
What many people do is place some lube on the bench and using there thumb and forefinger, spread a small amount around the case below the shoulder. Then, with the thin film still on your finger, slide it over the case neck which should place a small amount of lube inside a portion of the case neck. Your case is ready to be sized. As you continue this process, a number of cases going into the die at different rotary positions ensures that the case lube is uniform over the decapper / expanding pin. This trick saves the task of having to use a tool to ensure the inside of the case neck is uniformly lubed.
The following is an excerpt from "Modern Reloading" written by Richard Lee in regard to press strength..
"I take special delight in my part to end a trend of the manufacturers advertising how strong their presses are. Their claims were true. One particular press was 40 times stronger than need be to do the job. To point out how ridiculous it was, I turned down each of the support columns of a Lee Turret Press to 1/8 inch. I then sized a large magnum case with these skinny columns. A picture of this in all the shooting publications ended the "Mine is bigger than yours claims." The silly ads stopped."