How far is the northern border from the southern border in the state of Texas?

nice question

& one with many possible answers to boot

but basically

if all you really want to know is the length of the longest straight line that can be drawn within Texas from the panhandle to the Mexican border on the Rio grande

then that would be 806 miles & about 1270 feet

however if you mean to include the easternmost extension of the southern border of Texas

which tho not shown on many maps nevertheless definitely continues 3 leagues into the Gulf of Mexico from the mouth of the Rio grande at right angles to the seacoast

then that would be 809 miles & about 420 feet

of which about 30 miles actually run thru the territorial seas of the state of Texas

yet still entirely within the state of Texas

but if you want to know

or also want to know

how far apart the northern & southern borders of Texas are at their closest points

or in other Words

the length of the shortest line that can be drawn between them in the state of Texas

&

if you consider the northern border of Texas to include the northern border of west Texas & thus to extend all the way west to the Rio grande at Anthony

&

if you consider the southern border of Texas to extend all the way up the Rio grande from the sea as far as New Mexico

then that would be a mere 15 miles & about 1000 feet

& this last answer would have the added virtue of being a true north south line itself

so of all these possibilities

plus others too numerous & far fetched to mention

i would say the last mentioned is the truest possible answer

to the question you actually appear to have asked if not also meant

but if you meant something else

then please try again to say what you did mean as unmistakeably as possible

& i will try again to answer too