Hi
Don't expect to get good scans out of this (or any) scanner straight away, especially if it is your first foray into scanning. Many people will tell you that flatbeds don't cut it for 35mm, and that you need a dedicated film scanner, whereas they will suffice for MF. I find them fine up to a certain point for 35mm, and more than adequate for my MF needs (I shoot mainly classic cameras). Dense slides will present a challenge, so if you like shooting them, Astia or Provia will be easier than Velvia.
When I first started out with my 3170, my 35mm scans were rubbish, even for 6x4". My 6x6 scans were OK.
Now I can get a decent B+W scan from 35mm that prints well to A4. This is probably as large as you'd get from 35mm on this flatbed, and obviously it will also depend on the subject. My 6x6 and 6x9 scans print great up to A4 (paper size limitation on my printer!).
Scan in professional mode, and set scanning to low or off. Open the scanned file in you image editor, do your cropping and dust spotting, levels and sharpening. Sharpening on flatbeds is where you make or break your image. You need to experiment quite a bit till you get a process that works for you - straight USM, multiple passes of USM, or sharpening kit, etc.
I like to do a local contrast enhancement pass after spotting, cropping and levels. I save this as my "master. I then do a fairly agressive sharpen before resizing. Then another lighter pass of USM depending on the subject matter and film before printing.
Good luck, be patient and experiment a lot!