If I'm building a ramp that is 25 inches high and 25 feet long, what would be the surface length of the slope?

If by "25 feet long" you are referring to the horizontal distance then the distance along the sloping surface can be found from Pythagoras:

$${\frac{{\sqrt{{{\mathtt{25}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\left({\mathtt{25}}{\mathtt{\,\times\,}}{\mathtt{12}}\right)}^{{\mathtt{2}}}}}}{{\mathtt{12}}}} = {\mathtt{25.086\: \!655\: \!372\: \!483\: \!948\: \!9}}$$   feet

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25 feet  = 300 inches.....  and the slope is given by :

sin^-1 (25/300) = about 4.78°