Nancy, I have 3 citrus trees, Ponderosa lemon, Key lime and calamondin. Last year around this time I bit-the-bullet trimmed my Ponderosa. It bounced back without any ill effects. My advice to you is to look at it to determine where you think it needs pruning. Looking at your tree picture, on the LEFT I see a limb near the bottom, a space, another limb, a larger space and then the bushy top of the tree. I would trim all of the lower limbs below the first empty space, all around the tree. Just above the larger empty space is the first limb pointing left. A little below that limb it looks like you have Y. I would trim out the top on both sides of the Y at the left limb level, leaving the Y. Don't be afraid to hard-prune it. My calamondin it about 8 feet tall and 5 feet wide and is very full and bushy. I must prune it this year and I'll prune it hard.