Are there a minimum or maximum these escrow service providers may hold?
I've held funds for amounts as small as 0.01 BTC.
I've held funds for amounts as large as 140 BTC.
Each escrow provider will set their own limits (if any) on what they are willing to hold.
I hold all funds on paper wallets that are generated offline. I personally guarantee the security of the funds against loss, theft, and double-spending. Therefore, I won't accept funds for an amount so large that I can't pay the amount myself if something should unexpectedly destroy the paper wallet, or if the wallet (or contents) are stolen.
Any laws prohibiting them from going over these limits?
You'd have to check the jurisdiction where the escrow provider resides (if they are willing to tell you where they reside) to find out what laws (if any) apply to them. In general, most of the escrow providers on this forum are un-registered, un-licensed, and are ignoring any laws that might apply to any licensed, registered, escrow service. In many cases, this may be illegal in their jurisdiction, and there is a risk that if local authorities discover their "business", that they could be arrested and prosecuted. This could (obviously) result in significant delays in the release of any escrow that they may be holding at the time of their arrest.
I do not operate as a "business". I charge nothing for holding funds, and therefore act more as a trusted friend holding on to some bitcoin for you. I hold funds only for members of the bitcointalk.org community. Occasionally people who value my friendship and my trustworthiness will send me a gratuity to demonstrate their appreciation. These tips are graciously accepted.