Size cam. Lens for person?

I d like to shoot a person on a podium about 30 away. I d like a prime f 3.5 will do.
Don't need him to fill the too much.

Nikon 750

30 what away?

You do not state just how much of the person you want to have in the frame. Obviously a full-length shot will require a shorter focal length than a headshot. I'll assume you mean 30 feet away and not 30 meters, inches or miles.

Since you're using a full-frame FX body, you will need something in the 300-500mm range. Lenses in this focal length range will have apertures of f/4 or f/2.8. These types of telephoto lenses can be very expensive. If you need a lens just for a one-time event, then do not buy a lens. Instead, you should rent one. Renting a lens for a week will cost you a few hundred US dollars. This is a lot cheaper than buying a $3,000 ~ $11,000 f/2.8 lens.

For a head to knees shot at 30 feet you would need about 200mm on a FX body. The 70-200mm f/2.8 would work nicely for this. That's a $2800 lens if bought new. You can rent this lens.
If you need tighter, then the 300mm f/4 ($1.3k to $2k), 400mm f/2.8 ($11k), 200-400mm f/4 ($7k), or crop you images when using the 70-200mm lens.

Get a solid tripod also. Practice with all of it before the event. In the same light and room.

To isolate the subject at 30ft., you'd need around 200mm. To get a little more, 100mm would do better, so you can include parts of the foreground and background. The closest you can get with a single focal length lens might be the Nikkor AF DC 135mm f/2. If lighting is optimal, you can handhold the lens up to 1/125s. If poor, bring a tripod.

Prime lens are great, even short/moderate/long tele-lenses, but they limit what you can "reach" for; they maintain one perspective distance from the same spot you're located at regardless what you focus on… You would physically have to move closer or farther away from your intended object/subject to change distance perspective, or size of your intended subject/object in your frame.

The alternative is to use a "fast" zoom lens, where you can bring in (zoom in) or "push" the object/subject farther away from you without physically moving from your spot. Depending on the available lighting, you can use a Nikkor (Nikon) AF 80-200mm f/2.8D. Of course, there's also a more affordable lens, the Nikkor AF 70-300mm f/2.8D that affords you more "reach" and a variety of distance perspectives.

Here's some good news, with today's modern technological advancements and improvements in engineering and manufacturing, zoom lenses today are at an all-time high in focusing capacity and quality of images… Sharper images from "faster" (larger apertures) and have far better coatings in actual lens' glass and lens' barrel to eliminate distortions and abberrations that enable better resulting images than lenses of yesteryear.

Prime lenses are often regarded as "specialized" lenses for specific uses while zoom lenses are more versatile for different uses, or a variety of uses. Consider this when purchasing a lens.