1) A 100W transformer will handle your 91W with headroom for 1 more light if you wish. You can go right up to the transformer's rating without overloading it.
2) You wire the lights in parallel. This means that you tap into the power feed at each light. I'm sure the wiring diagram will explain this perfectly.
3) You're drawing about 8A here and with 100' I'd use 12 ga wire to be on the safe side. Make sure you use the special outdoor rated low voltage wire.

Gee 12 gauge wier? See my post on LED lights, if we had low output LED fixtures we'd see a reduction in current and power sufficienty to wire with "bell wire", and think of the power we'd save. As I noted in the mentioned post, I find the current crop to have a strange light "color", that needs to be corrected and I'm sure it can be. Think of the energy it would save.

A loop configuration as you suggested has little advantage over two (or more) runs from the transformer each to half (or less) of the lights. Running the wire each way, half-way around the "loop" but not connecting those ends where they would meet, is the simplest way to cut down the wire losses. It puts half the current in each wire and the current runs half the distance in each wire.

Doing this would probably let you use #14 instead of #12.

As far as color, LEDs were originally monochromatic and limited progress has been made in making them have more spectral components (blue white). Thus they do not bring out the visual differences in the things they illuminate nearly so well as the broad spectrum of incandescent.

Fluorescent bulbs have some of the same problem but the phosphors are chosen to re-emit some of the light as a somewhat broader spectrum. This is why fluorescents do not make good reading lamps and some of them are hard to see by even when their "equivalent wattage" should be adequate.

So I would run one wire in one direction and one in the other direction around the deck attaching half (or 6 to one and 7 to the other)of the lights to each? And the leads to those wires would be connected to the same terminals at the transformer? Is this the parallel hookup that was discussed in the first reply? I know the 100 Watt transformers have only A B and C terminals , the C apparently being for a light sensor hookup.

Is there a consensus on what gauge wire I will need then? 14 or 12 gauge? I'm not really interested in LED's as I just don't like the look of them.

Thanks for all the help!

Randy

Yes, one cable or pair of wires to 6 lights and one cable to 7 lights, and the lights are in parallel. Each light connects across both of the wires that run past it without interrupting the flow to other lamps. Two terminals on the transformer are used and each terminal has two wires on it, one wire from each cable to each terminal.

Using #12 gauge wouldn't hurt, but is probably not necessary. With #14 I calculate a 5.7% voltage drop, and with #12 it is 3.7%. A rule of thumb is to allow 5%. This is at the last of 7 lights spaced 8 feet apart, operating from a 12.0 volt transformer, 7 watt bulbs, using room temperature wire resistance (not the NEC table at 75 deg C).

With 5.6% drop, you might be able to tell the last light wasn't quite as bright as the first if they were side by side, but not across the deck.

If you put in a full loop of wire there would be scarcely any difference from using the two sections.

By comparison, if you ran all 13 lights on one pair of #12 wires unlooped, the drop at the last lamp would be almost a volt and a half, a whopping 12% loss and you would see the difference in brightness.

Thanks for all your help!!!

Check out my photo tutorials and video tutorials regarding deck lighting.
http://www.moonlightdecks.com/How_To_Wire_Low_Voltage_Deck_Lights.php

http://www.moonlightdecks.com/Videos-About-How_To_Wire_Low_Voltage_Deck_Lights.php

Here is a link that might be useful: Video tutorials on deck lighting