Most listings are in the $10's per square foot per year. The Leesburg submarket is going through a a rise in rates. In the last 3 months, the median rate went up $2 per sqft per year. That's a 11% rise in rent.
In the last 90 days, 11 listings have been posted online, which means listings are probably leasing quickly. You'll want to be prepared to make an offer immediately. About 9% of the available leases in the Leesburg are subleases, so you will probably need to sign a longer term lease with the landlord directly. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,350 sqft. Most buildings in this market are approximately 1 floors. Most buildings in this area were built in 1980 and are Class A.