Outstanding, thanks. This has been a super helpful conversation for a newbie to the world of water pumps. I told my wife I was looking for a water cannon that would just shoot the water over the barn onto the garden 500' away and she just looked at me with the rolled eyes and said, that ain't happenin'. She actually considered the possibility I was serious.You do not have to use the same size pipe for an entire run. In some circumstances, it is common to use differing pipe sizes, although that is usually more often with elevation changes or other things different from your circumstances.
Water flowing through pipes is subject to friction loss, as the water bumps up along the internal wall of the pipe as it moves along. Using the same pump, if you pump through 50 feet of pipe, the velocity of water coming out the end of the pipe will be much greater than if you pump through 500 feet of pipe.
But, as ponytug also pointed out, it may be disadvantageous to use a gas powered pump moving water directly from the pond all the way onto your garden and out the sprinkler heads. Every time you wanted to water the garden, you would have to go start the pump. Then let the pump run while you water the garden, then go shut it off. If it were me, I would want something requiring less frequent attention. Also, the pump would likely have a lot more capacity than just running a few sprinklers-- so you would be burning gas somewhat inefficiently.
As an alternative, the gas pump could (quickly) fill a holding tank near your garden. Then you would shut the gas pump off. Depending on size, that amount of water might last several days or more without firing the gas pump up again. An electric pump could take the water from the holding tank, pressurize it, and run the sprinklers. That sort of arrangement could also be operated on a traditional irrigation timer. An advantage would be, depending on size of your holding tank, you might not need to run the gas pump for several days, or maybe a week or more.
Finally, if you want to run sprinklers directly from the gas pump in the pond, lots of calculations of pipe size, friction loss, gpm per sprinkler, etc. are needed to ensure success. If the gas pump just fills a holding tank, you need not bother with almost any of that. Because the gas pump will push water to the holding tank-- the only variable being the speed it achieves to fill it.
Hope that helps.