I am pouring a 24x25 reinforced slab, and I am hoping to do it all in one pour. The only real problem I can see is that screeding will be nearly impossible, without a 25 foot screed board. Is this possible?

There are many different ways to do this.

The first thing to determine is how long of a "screed Board" you want to pull. The average is 8' or 10'. Lets say we are using the 10' sceed board. Cut some 3/8 rebar and use them for height pins inside the forms. Making point to point squares. Measure in on all sides of the forms 9' and keep all squares 9'x9' or less. Hammer the pins in at those points and make sure they are level with the finish height of the floor. This will allow you to "wet bench" a height to pour. To pour, start in a corner and screed out at a 45 degree angle until you run out screed length. Now pour fresh Crete in strip wide enough to screed form pin to pin/ pin to outside form.

Fill inside that square and treat use the concrete strip as a set form. Once you squared that out pound 3/8 rebar down 2" or more. Repeat this procces as if you are pouring mini pours. You need a light touch, so as not to dig in the wet bench with the screed board.

Another way, if you have a roof over head and its level pick points and cut a 2by to fit from ceiling to finish floor. This story pole will allow you to Wet Bench whever you want. Cautaion, check ceiling to see if its level. If not use a transit to set shims to adjust for ceiling truss deflction. If its an open ceiling truss scab short 2bys on the side of the truss in the grid pattern to put the story pole up aganist to wet bench below that point.

If a transit is not avaliable then set inside rebar pins first as stated above. From the top of those set pins measure up to ceiling and cut 2x4 to hit ceiling and top of set pins. Shim where nesscarry on ceiling for story pole. This added set up can save your butt in case a person trips into a pin and knocks it out of wack while pouring. The story pole that was plumbed up from that pin is handy to check the wet bench height.

On Inside garage pours I use meatal screed rail and wet bench to that. To use this you will need to reinforcement bar/wire in crete because of the screed rail that is 4" high or 6" hing depending on the floor thickness pour. The metal screed rail also acts as a conrol joint for any cracking and if it does it will hairline crack in a nice straight line at the screed rails.