Beltre had proven himself for a long time before the Sox got him, and was only injured (he still played 100 games that year) for one season prior to his contract, and was mostly an above average hitter in Seattle, a place that kills offensive stats.
He was also a stud in the field, and was a totally low risk, most likely high reward type of player. Sizemore COULD be low risk for the right price, but the likelihood of the high reward just isn't very good with him. He hasn't been effective or able to play full-time for 3 years, so I'm not as for this idea as I was for Beltre. However, with that said, the reward for a 100% Sizemore is very very high. In his best days, he was a great offensive weapon. Lots of speed, lots of power, good on-base ability, from 2005-2008 he hit for a 128 OPS+ (.281/.372/.496).
I'm open to it for the right price, but he's not a good bet to produce the high reward like Beltre, or even close.