If I run a 100 foot phone wire around the house to install another jack, will that lower my DSL speed.Is there a maximum length for phone wire, before it lowers the signal?

Did u mean ADSL? Yes the length of wire does matter because also anything could happen in between (a few users had their dogs chewed the wires, i'm not joking).

Having said that, every case is different. I think u should deal with the problem IF it comes. There could be other reasons for slow connections.

The maximum is 100 metres. Approx 300-ft.

You can not use a telephone cable as short as wall outlets are located in places near sources of heat, angles, etc. and look at our house you can not set the computer near wall outlets. In addition, we would have wires all over the place, making great opportunities to travel and get hurt, or put together tables.

Phone wires travel from the Central Office, to (and through) your home as a twisted pair. The pair of wires is twisted to cancel cross-talk (foreign noise induced on the pair). The quality of your connection is dependent on a SNR(signal-to-noise ratio). Using a long line-cord is not the best idea, but if you have no issues, give it a try. I would get a baseline SNR near the jack, and compare it to the SNR on the long line-cord. It would be in the modem transciever statisitcs under SNR, or Noise Margin. Make sure you are comparing apples for apples though... make sure your downstream and upstream data rateson the second test match the rates on the second!

DSL speed is not depand on wire length , speed slow downd when number of user used dsl Connection .

should not affect the speed really over 300ft could see a small drop in speed but hardly noticable

When we talk about length limits for DSL, there is the twisted pair from the Central Office to the modem, and the two or more twisted pairs from the modem to the PC. 25000ft is the untreated theoretical length limit to the modem. The 300ft limit would be for Ethernet.

I beleived there will be a very small effect on the speed.