R.M.'s suggestion is good. . . . It sounds like it might be a PDF if you can't adjust the print size when you sideload it (using the USB cable), which is why you'd need to put the word 'convert' in the subject of the email you send to Amazon. If it's some other format, say a Word file or something, 'convert' is not needed as Amazon will do it automatically if it's a convert-able format. (Kindles can read PDF natively so you need to make it clear if you want it converted or just sent along.

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Then attach the file. Make sure you know your Kindle's 'send to' address -- see the 'Manage Your Kindle' page via your account at Amazon. And make sure you're sending from a 'whitelisted' address -- also something you can check/change at MYK.
Note that if it is delivered via 3G you may incur a small charge. You can prevent that by using the same send to address except make it @free.kindl.com. Or by going to MYK and changing the charge limit for personal document delivery to Zero. That way they won't deliver every via 3G but it will come next time you connect via Wifi.