Greenhouses come in a variety of sizes and shapes. For years, nurseries constructed greenhouses that were long with a rounded top, faced the long sides to the east and west, then covered the whole thing with fiberglass or plastic.
This design does not make a good solar greenhouse!
The broad side should face the direction of the sun in winter, which is south. If the broad side faces east and west, it will get little winter sun and lots of summer sun, which is just the opposite of what we need.
A rounded roof covered with plastic or fiberglass glazing will let lots of hot summer sun in but will gather little of the low winter sun. The summer sun gets quite high, to an altitude of 80+ degrees. That's almost overhead, so it's not the place to put glazing!
A solar greenhouse can be about as long (east to west) as you want it to be, however it shouldn't be very deep front to back (south to north). Fourteen feet deep is plenty for an attached solar green house. Anything deeper than that won't get much sunshine to the back of the greenhouse in winter. To get sunshine to the back, you'll have to add skylights or clerestory windows, both of which add cost to the project and provide places for potential roof leaks.
While you wander outside trying to visualize your greenhouse and mark off possible sizes, think about how you want to use it. Will you be starting seedlings in the spring? Will you grow lots of indoor-type plants? Do you want some miniature citrus trees? Will you be raising winter crops of lettuce, tomatoes and peas? Do you want all of the above?
In general, it's not a good idea to raise plants that need to spread out. Most melons and some kinds of beans have leaves that consume a large amount of space and will fill up a small greenhouse quickly. It's better to plant these outside.
Most herbs grow well in a greenhouse. Leeks love a winter greenhouse and so do tomatoes. Most cool season crops do well if the greenhouse is managed correctly and not allowed to get too hot. Most plants will grow slower than they would in your summer garden due to shorter days. Supplemental lighting will help to stimulate their growth.
One of the best books to help you learn about greenhouse gardening is The Solar Greenhouse Book edited by James C. McCullagh. The book offers detailed information on design and construction as well as planting. The majority of the information, however is geared to northern climates.
Next month: how to store the heat you gather from the sun and more about plants.