SAN ANTONIO — In a state where everything is big, the 23rd Congressional District that hugs the border with Mexico is a monster: eight and a half hours by car across a stretch of land bigger than any state east of the Mississippi. In 2014, Representative Pete Gallego logged more than 70,000 miles there in his white Chevy Tahoe, campaigning for re-election to the House — and lost by a bare 2,422 votes.