= Solution
The supplied confinement criterion alone fixes a product, not a unique current or machine size. Choose a representative ion density $n_i=10^{20}\,\mathrm{m^{-3}}$. Then
$$
\boxed{\tau\gtrsim\frac{10^{20}}{n_i}\,\mathrm s\simeq1\,\mathrm s.}
$$
For $n_i=10^{19}$ to $10^{21}\,\mathrm{m^{-3}}$, the same criterion gives $\tau\gtrsim10$ to $0.1\,\mathrm s$. These are illustrative density choices, not additional source data.
For a current estimate, assume a large-aspect-ratio circular tokamak with major radius $R_0=3\,\mathrm m$, minor radius $a=1\,\mathrm m$, toroidal field $B_\phi=5\,\mathrm T$ and edge <tokamak safety factor> $q_s\simeq3$. The geometrical winding relation and <Ampère's law> give
$$
q_s\simeq\frac{aB_\phi}{R_0B_\theta},\qquad B_\theta(a)\simeq\frac{\mu_0I_p}{2\pi a},\qquad I_p\simeq\frac{2\pi a^2B_\phi}{\mu_0R_0q_s}.
$$
For the stated assumptions,
$$
\boxed{I_p\simeq2.8\times10^6\,\mathrm A,}
$$
so toroidal plasma currents of order megaamperes are a natural scale. As a consistency check at $T\sim10^8\,\mathrm K$, quasineutral singly charged heavy hydrogen has total thermal pressure $p\simeq2n_ik_BT\simeq2.8\times10^5\,\mathrm{Pa}$. With $5\,\mathrm T$, $2\mu_0p/B^2\simeq0.028$, so the assumed magnetic pressure exceeds the thermal pressure. The particular current depends on geometry and field-line pitch; it cannot be inferred from the confinement product alone.
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