I was just given
this granny fork as a gift and I love it.

It's $11.45. At 7", it's the perfect size for me (I'm 5'9" with pretty average size hands). Before getting this fork, I just used a regular fork when slicing meat. This fork is a vast improvement and I'm very happy to have it.
You are going to pay around $80-$100 for a Wusthof fork. I'm not saying this is a bad idea or a waste of money. That's up to your preferences. But I think the price difference in price is mainly buying a fork that matches the knives. The construction of the Wusthof may be better (i.e. stamped vs forged metal, etc.). But, unlike a chef's knife, I don't think these factors will have much affect on usability. They may feel different in your hands because of differences in weight, shape, etc., but that's something the user would have to experience themselves.
In short, I can't help you with the straight/curved decision. I think 7" is a good fit for me. Personally, for $80-$100, I'd rather have a new knife than a fork. But again, that's a personal decision. I have some Wusthof knives, but don't care about mixing brands and quality levels.