I make doll quilts for Operation Christmas Child (to pack in a shoebox with a doll and other goodies for a little girl). They are generally about 18x20, give or take. It's a great way to practice certain blocks without a big commitment. It gives me a chance to start something and finish it in fairly short order .
You can also donate to almost any kind of charity where your heart is inclined to comfort people, from babies in a local NICU, to veterans, to children in crisis (ask your local police dept), to cancer patients. The sizes will vary depending on the recipient and you will feel good in creating something that will make someone else feel special if you don't have use for it but still enjoy the process.