my biceps are quite large compared to the rest of my body and I really want to tone them down.
Theyre also quite soft when I flex and squeeze them.
I can do about 40-50 reps with 20
and about 10-20 reps with 35 pounds.
what would be a good bicep workout and right WEIGHT to slim them down?
thanks

Sounds like those aren't biceps but lots of fat surrounding them.
Cardio lots of it ~ change your diet to achieve your goals ~ avoid direct arm training ~ more cardio ~ morning afternoon evening ~ Total body training 4-5 times a week!
Good luck. You have a high body-fat % based on your description. Expect to after losing the fat around your whole body to also lose some muscle mass.
Circuit Training, Low rest times 15-25 seconds every set or lesss 5-10 or non at all!
There are some savage fitness ciruits but you can also find something in the Crossfit scene or in MMA training etc.
Change your eating. Basically throw out whatever you are doing in terms of eating and start from scratch.
Stop direct arm training.
Sprinting jumping rope swimming - do all of the above and don't just pick 1. Do 2-3 different types of cardio!!
OR RUN IN THE SAND ~ courtesy of Laris and the sandfiends! I've ran out of my free promotional cardio bunny ears but I'm sure soompi will open up a store and start selling them for me after we sign a partnership agreement!
Good luck ~ learn to love cardio and dieting! Pushups and pullups! tons of them! aim for 20+ a set for pullups and 100 pushups a set.