We assume that you hope to get edible pecans or walnuts. Unfortunately the odds are not good for success in Maryland. Traditionally black walnuts are more successful in Maryland than the English walnut, but the new potential for thousand canker disease which has recently been found in Maryland makes all walnuts a bad selection. http://na.fs.fed.us/pubs/palerts/cankers_disease/thousand_cankers_disease_screen_res.pdf  Hardy pecan trees can be grown in Maryland, but the odds of getting edible pecans is low due to weevils and other insect problems. We do not have a list of recommended vendors. A good size to plant is about two to six feet tall and the best time to plant is in early spring before new growth starts.  vw