Is there any limitation on the monetary size of transactions to be paid through BitCoins? I conduct a consultancy business where payments from clients can vary between about $5 for White papers to $15,000 for a major consultancy assignment fee. I have not been able to find out this information from the website.
There is no technical limitation. However, to perform a $15,000 transaction, you need $15,000 worth of Bitcoins. There are times when it's hard to acquire that many Bitcoins without moving the market in unfavorable ways. It's worse if you need to cash out. However, if you're willing to buy and sell the Bitcoins slowly or under favorable market conditions, the transfers are not a problem.
$15,000 is not a huge problem right now. $300,000 definitely is. Just remember, if you have to buy bitcoins with dollars and then the other party has to sell them for dollars (or another currency), you'll take a sextuple transfer hit. You'll pay two half-spreads plus two conversion fees. Then, as you buy, you'll move the market up. And as you sell, you'll move the market down. This is a huge issue with Bitcoin today. Just the spread and conversion fees add up to about a 2% loss, assuming prices don't change -- that still beats credit cards.
Bitcoin transaction fees for transferring Bitcoins are based on the size of a transaction in bytes, not the number of Bitcoins transferred. What makes a transaction "expensive" (though we're usually talking a few pennies if even that) is having to "gather" funds from a lot of sources or "scatter" funds out to a lot of different destinations. So your $15,000 transaction will likely cost less than a penny.
So in sum, moving the Bitcoins from one account to another will probably be less than a penny. But in most applications, you have to factor in the cost of buying Bitcoins on one end and selling them on the other.
In the most recently-mined block at the time I wrote this answer, there was a transfer for about $16,000 worth of Bitcoins. The transaction fee was about a quarter of a penny. And currently you can process transactions without paying any fee if you're willing to wait an extra 30 minutes or so. (Because many miners don't care about transactions fees and will include any valid transaction so any valid transaction will get into a block eventually.)
The only technical limit on the number of bitcoins you can send in a transaction is that you can only send as many bitcoins are you are in possession of, and that there will only ever be 21 million bitcoins in existence.
If you have bitcoins, and someone else wants them, there's no problem sending as many as you like.