Being plus-size can to some degree potentially affect ultrasounds, fundal measurements, hearing the fetal heartbeat, and feeling your baby move — but these are not big problems to worry about.
With ultrasounds, your level 2 ultrasound should be done at 18 to 20 weeks. If the exam doesn't produce adequate results, it can be repeated later. But my experience has been that the exam usually works fine for plus-size women.
With fundal measurements, even though an individual measurement may not be accurate, the trend of measurements during pregnancy will still show a consistent pattern of your baby's growth. The only potential issue is if your doctor feels your baby may be macrosomic (nearly 10 pounds or more). An ultrasound can provide a more accurate idea of your baby's size.
With a fetal heartbeat, I've found that sometimes a plus-size woman's pregnancy needs to be a bit further along, in terms of weeks, before I can hear her baby's heartbeat. But then again, I've had the same experience with thin patients. Of course, it also depends on how good your healthcare provider's hearing is — my nurse practitioner can usually hear the heartbeat before I can! I resort to an ultrasound machine relatively quickly if I don't hear the heartbeat in the first few minutes. As your pregnancy progresses, it will be easier to hear your baby's heartbeat.
As for feeling your baby move, some plus-size women do feel fetal movement later, but other factors are involved — such as whether it's your first pregnancy or whether the placenta is positioned in such a way that it's cushioning the baby's movement. So no one can say for sure whether the delayed perception of fetal movement is because of your being plus-size.
None of these are issues to sweat about. Every woman's pregnancy is different, no matter what her size, so don't let these concerns make you anxious.