Learn English – Is it correct to say ‘small streets’

I cannot speak to English elsewhere, but in the US there would be no need to use size at all—‘small business’ is the term.

If you are concerned that bare small suggests inferiority, quite the opposite is true. ‘Small businesses’ are generally recognized—or at least recognize themselves!—as the engine which drives the American economy, and there is a large Federal agency, the Small Business Administration, dedicated to their support. They have indeed something of a mystique: ‘small business owners’ regard themselves as a distinct class, superior in efficiency, energy and entrepreneurship to the bureaucratic managers of large corporations.

Of course the US notion of ‘small business’ may be substantially larger than that which obtains in your country. The SBA standard varies from industry to industry, but by and large it is less than 500 employees in manufacturing and less than $7M annual revenue in non-manufacturing industries.

Considering the audience aren't native english speakers, one or two people not too firm in the language might be irritated by "moving on", so make sure you express what's going on in the email's content, I suppose.

"Moving on" by itself is a very good choice, though, especially as it sounds a tad more positive than plain "Leaving", as user8543 mentioned.