How can I calculate the input resistance of this BJT Differential Amplifier circuit
Looking between the two bases, the small signal resistance will be

2 * beta * 1/gm

At 1milliAmp, the 1/gm will be 26 ohms.

If beta = 100, the small signal reac (from base to emitter) will be 26 * 100 = 2,600 ohms.

Measures across 2 bases, total is 5,200 ohms.

This ignores all other effects, and ignores any effects of parasitic capacitors.