brod poor you. Midwives sometimes don't think how much they can panic people with poor explanations!
Firstly, merk is totally right - measuring with a tape is very subjective and a 'guesstimate' at best regarding the size of baby. Position of baby, your abdominal length, body fat, size of the lunch you just ate and who measured you can all skew the measurement. Do you have a personalised growth chart? They're slightly more accurate as they take height/weight into consideration but that's all.
A lot of women measure small or large for dates, and it's policy to send all of them for growth scans. The vast majority have perfectly normal sized babies, measuring is basically a very crude screening test designed to pick up those few who do have issues with growth.
If I were you I'd keep away from google, the chances are you'll have a lovely scan and everything will be totally normal. If it turns out she's a big baby they might check you again for gestational diabetes, and if there's a lot of fluid they might do another anatomy scan to make sure there are no problems with her digestive system or the placenta. To reassure you, extra fluid is quite common and often no cause is ever found, you'd just be monitored a bit more closely; gestational diabetes can usually be managed well with diet. But like I said, most likely outcome is a very normal sized baby in there!
Not sure what the midwife means by 'engaged position', at 28weeks she probably just means that the baby is cephalic (head down). Even if the head is in your pelvis at the moment it could easily pop out again, the head is too small at this point to engage properly!
Sorry you've had a fright, hopefully your scan will come through quickly and reassure you that everything's normal xx