I got a lot of questions from growers this time of year about their plants being kind of small. Right about now your plant should be vining but that vine may or not be laying on the ground right now. Cool temperatures will slow the plants down. Until night time temperatures get into the 50s your plant isn't going to grow a lot.
As long as your plant isn't way behind don't sweat it. At this time of you want the roots growing. The salad will catch up by the second week of June and you will have more vines then you will know what to do with.