Short answer first: "Before" inflation the universe was extremely young — well before first second: t ≲ 10^-36–10^-34 s (the Planck era is earlier, t ∼5.4×10^-44 s). The small patch that later inflated to become our observable Universe had a physical size somewhere between ~10^-35 m (Planck scale) and something like 10^-26–10^-30 m depending on the inflation model and assumed number of e‑folds.
During inflation the scale factor grew exponentially: a(t) ∝ e^{Ht} with H approximately constant. For typical GUT-scale inflation (energy scale E_inf ∼10^15–10^16 GeV) one finds H ∼ 10^35–10^38 s^-1 (i.e. an e‑fold timescale 1/H ∼10^-38–10^-35 s). Different assumed E_inf (lower or higher) shifts H accordingly.
Useful relation and post‑inflation behavior: H^2 = (8πG/3) ρ_infl, and for an inflaton energy scale E_inf (with ρ_infl∼E_inf^4 in natural units) roughly H ≈ E_inf^2/(√3 M_Pl). When inflation ends (reheating) the expansion ceases to be exponential and becomes power‑law (radiation era: a∝t^{1/2}, H≈1/(2t)).