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
The capital of a state that shares a southern border with Texas is Mexico City, which is the capital of Mexico.
Oklahoma shares most of Texas's northern border.
Texas and Arizona
Nebraska.
Texas.
Oklahoma City is the capital and largest city in the U.S. state of Oklahoma, the state that shares its long southern border with Texas.
Oklahoma
It is a western state along the northern border.
Texas at Southern border
Lincoln is the capital of Nebraska, a state whose northern border is South Dakota and whose southern border is Kansas.
tulsa
Texas and Louisiana