i've been with bigger girls in the past and this is one of the very few scenarios where i will honestly say that having a longer dick helps as you can reach into her albeit any fat that is in the way (surely no pun towards big women but let's keep it real here). i was with this bigger gal in my younger years and missionary position worked wonders. as
@Holly Doors mentioned, just spread her legs wide and slide yourself right in; trust me she will feel you for sure.
as for doggie, if she is able or willing to arch her back and stick her ass out and high, you can also give it to her pretty good too. if there is still some junk-in-the-trunk getting in the way, one trick that i've learned is to palm a nice chunk of each of her ass-cheeks and push them forward towards here. this movement will not only spread her ass-cheeks but also expose her pussy more readily and you can slide you way in very easily since you've pushed all the junk forward. trust me on this one, i've done it and have given her a very nice, deep fuck.
finally, just talk with her and be patient as well as open to trying different positions until it works. some of the best sex i've ever had overall has been with women who were a little bit on the bigger side. the skinnier women are easier to fuck but not necessarily better by comparison. good luck and have fun while you give her a good fucking!
for the record, my wife is one of the skinny ones but i wasn't after her body, i was after her super-sexy-slutty brain.
