I just made one for my son, it was definitely not hard, even though it was my first time sewing, but most definitely a labor of love as it was really tedious.
The way you want it to be is that it covers you with very little overhang, so that's how big you need it to be.
For weight, it is said that it should be 10% of your weight plus 1 lb, i.e. if you weigh 100 lbs than 10% of your weight is 10 lbs plus 1 lb would make the ideal blanket weight for you 11 lbs. Some people like more weight though, so you have to keep that in mind.
If you're interested in making your own, there are many YouTube videos explaining it in detail. I think I even came across one that was 'no sewing involved'.
Good luck!