have her spread her legs more and see if you can get your knees under her lower abs, then you will be more in place, also if this doesnt work, try having her assume the position on a couch or bed with you standing, it should give you more room to manuever and achieve.... well you know.
As suggested in this forum for 1a doggy go for it! but be aware that as with all anatomy not every lady is built the same. There are great differences in size, shape, appearance AND position of a vagina. Some are positioned forward (north oriented) some rearward (south oriented). My G/F is a southerner and ideal for 1a but has to put the ankles on my shoulders for "modified" missionary. Try it all but settle for what is best for you both.
one way in which you CAN do this is lay flat on the bed.. I know this is not the traditional way of doing it so bare with me..
Lay down flat on the bed. Have her sit on top of you in a reverse direction. and viola... doggie stlye but not..
I'm really not sure how to help you with this issue. I do doggy style and have done it many times, but vaginal entry really does depend on the size & shape of the penis and the angles involved. If your issue is that your penis can't make far enough to gain entry, then you may need to modify the doggy style and have her lay face down on the end of the bed so that her knees are on the floor and her butt is jutting out. Have her then spread her butt cheeks to give you as clear a path as possible. You then kneel behind her and place your erection into her vaginal. Hopefully this will give you the 'clearance' that you need.
Not wanting to kid you on this, but losing some wieght really would be a better answer (something to think about).
Some men may have difficulty with doggie style as it requires the penis to be a little longer than regular positions as the vagina is farther away. If the penis does not reach or falls out you may have to modify your positon. The woman should try putting her head all the way down to the bed and tip her butt up - this may tilt the vagina closer.