Three or four hundred words should be plenty. That's long enough to look reasonably substantial, to fill a computer screen, and to contain three or four links in the text without looking silly.
If you're writing articles for other websites, as I discussed last week, what should they have in them to be really effective? The length of an article is a key consideration. If it's too short, the search engines won't take it seriously, and the website you're targeting won't find it as interesting to run. If it's too long, it's going to cost you too much to get written to make the exercise worthwhile (and anyway, you could surely split it into several shorter articles).
Three or four hundred words should be plenty. That's long enough to look reasonably substantial, to fill a computer screen, and to contain three or four links in the text without looking silly. Most decent freelance technical writers would probably produce you an article of that length from (say) a brochure, for a couple of hundred pounds, so setting up an "article a week" production line needn't be out of the reach of most marketing budgets. Your writer can also re-work articles into new versions, perhaps concentrating on different features or benefits of a product, so you needn't have to come up with a new source of original information each time you want an article.
I wouldn't provide the article for the external website all marked up with HTML code, as their content management system will almost certainly take care of that and it would therefore make their job difficult. However, always include the links you want within the text pre-marked-up, like this:
…The product can be used in conjunction with <a href="http://www.bmon.co.uk/guides/4-20ma-aerospace-widgets/">4-20mA aerospace widgets</a> for greater efficiency…
…and it's worth sending a covering note with your submitted article to ensure the website which will publish it knows that these links are important background information for the article and shouldn't be removed.
Finally, ensure that the other site gives full credit to your company as the author of the article, and links to your site from the company name appropriately.