Frame size is an out of date concept. Some brands use numbers and others use letters, which makes both meaningless. Use stack and reach instead. Saddle to bar drop is also a meaningful measure. These days, frames increase stack by 20-25mm between sizes. Reach comparison is a bit tricky. If two frames are compared that have a 20mm difference instack, subtract 6mm from the reach of the smaller frame to get the difference in reach at the same stack height. I've always used an 8-10cm saddle to bar drop, which is a lot for a 168cm rider.
Moving the saddle back will reduce the pressure on your hands. I like to have a little upward angle to the area of the brake hood where my palm rests. I use a digital level to measure and usually have about 8 degrees up. Level hoods feel sloping down to me and cause pressure at the crook of my thumb.
I've had left hand numbness problems on occasion, but it's just gone away on it's own. Can't explain that, other than I took almost 8 years off the bike and the problem ended after a couple of 4-5,000 mile years. I rode 5700 miles last year. I've got about 18,000 on my new pair of knees.