Answer: In general, beans need full sun to grow well. They also need soil of at least average fertility and ample water to help them "size up".
You may want to check with your UMASS extension about running some basic soil tests and then amending your soil as needed based on the results of the test. It is also a good idea to add organic matter such as compost or rotted leaves or aged stable manure and bedding to help feed the soil and improve the structure of it.
To some extent, the size depends on the variety (French filet beans for instance are still skinny even when overgrown) and on how mature the beans are. If left too long on the plant, the beans will thicken as the seeds inside them swell but then they will not taste as sweet any more -- they will also have a nasty texture if left on the plant too long. Most variety descriptions provide the optimum size at which they are best picked. You can also pick them smaller, of course.
Burpee offers a number of excellent varieties, but my personal preference for fresh eating is Kentucky Blue pole bean; for bush beans, I like Tenderpick fine but my favorites are the filet beans. You might plant a selection and see which ones you like best.
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