I don't load magnums, but I do have a Lee Universal decap die and measured it.
The I.D. is .565 inches.
The max length case it can handle is 2.920 inches, maybe a touch more, assuming you have it set up with the shell holder touching the die.
Looking at the dimensions for a .338 Win. Mag., it should be all right.
It will be close on the diameter at the belt though.
Maybe the folks who had trouble got a "small" die. I don't know what Lee's acceptable tolerances are.
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