Welcome to the forum!
I believe that there was a post last year that was similar to this - somewhere on here.
The largest one I have removed was an 8 pound basketball sized lipoma. The dog was a golden retriever that was about 11 years old. The lipoma was so large it was 50% the thickness of her hind end. The owner decided on surgery because the poor dog kept falling over due to the weight. The dog did fantastic. I have not ever had problems with excess bleeding with lipomas. Even without the use of a laser, typically they can do very well. (the 8lb one was done without the use of a laser).
Lipomas are benign fatty tumors. When you look at them they are almost all fat with very little blood supply. In fact if during surgery, large blood supplies are found, then the concern of malignancy would be raised. The largest complication I have had with giant lipomas is closing the space that the lipoma once filled. It takes a lot of time, often removal of excess skin and for giant lipomas, the addition of penrose drains.
As with all major surgeries, platelet count, protein level and bleeding times should be done preoperatively.
You said the tumor was behind (caudal) to the shoulder? Not under the arm pit correct? Under the armpit is a much more difficult surgical area. If the mass is behind the shoulder where the ribs are - this is actually a fairly good surgical area. Post operative exercise restriction can help with minimizing post operative swelling and drainage.
To date, I have never had excess bleeding issues with giant lipoma removals. Of course every case is different. If your vet and you are concerned about the surgery, then perhaps consultation with a local surgical specialist may help.