I'll be in Tuscany for about 10 days and plan to get an Italian sim card for my iPhone SE. The primary intended use will be for data. The goal is to use mapping and GPS apps for navigation while driving. I might do some web browsing and other use of data also. Might also do voice over IP for phone calls. I need to decide which sim card to get (Gigabyte capacity). Would 10 GB be enough? Any advice of which sim card to get would be appreciated.
Most likely, 10GB would be far more than you need for ten days. For driving GPS, in theory you don't need any data at all; you could download the Google Maps "offline area" for Tuscany to your phone while on WiFI and not need a SIM. Having a SIM will make somethings easier, though, and SIM cards are fairly cheap nowadays, so it may still be worth getting one.
For reference, in May I used 2GB of data in 17 days in Europe. I did try to conserve data by leaving data OFF while driving the one day rented a car (I used the "offline" map most of the time), but I used mobile data while navigating on foot all the time. I made some calls with Google Hangouts (seems to use about 2MB per minute - but if you have WiFi it won't use any mobile data). Unless you're planning to stream videos on your phone, I doubt you'll come anywhere near 5GB in ten days.
Data capacity is not decided by SIM card but by what package you are on with the network provider. SIM cards are not storage cards you simply choose what package to opt for based on your requirements. Your only requirement is that your phone is unlocked in order to be able to use a different network provider to your current one. When you purchase your SIM card in Italy you can decide how you want to manage your allowance, either pay for a small allowance and then top it up according to your requirements or it may be cheaper to opt for a monthly package.
Last fall I was on Sardinia and Corsica driving around for two weeks. I then as on the boot for an additional 10 days. Not only did I constantly use mapping for directions and routing but I also used a GPS route tracker (an app notorious for eating up data). I had them on constantly and also used the iPhone for research, photos, etc. At the end of 22 days, I used a hair over 2 gigs of allowed data. The TIM tourist card comes with 3 gigs, Vodafone with 2. I think either one would suit your needs.
Thanks for the useful advice. I will probably end up getting a tourist SIM card at one of the TIM locations in Firenze.