[jcklwy]

In a reaction between mg and hcl, the magnesium is designed to be the limiting reagent. Why is this? Is the solid always the limiting reagent?

[Borek]

In a reaction between mg and hcl, the magnesium is designed to be the limiting reagent.

Not necessarily. You can design an experiment this way, but it is not a must.

Most likely the experiment you have on mind was designed this way for some specific reason. Hard to tell not knowing details.

Why is this? Is the solid always the limiting reagent?

No. For example, if you use solid CaO to absorb CO₂ from the flowing gas, you want to have an excess CaO to make sure none of the CO₂ passed through.

[kriggy]

Its probably practical reasons: separating solid MgCl₂ from HCl solution is fairly easy. Separating solid MgCl₂ from solid Mg chunks is fairly complicated

[Borek]

Its probably practical reasons: separating solid MgCl₂ from HCl solution is fairly easy. Separating solid MgCl₂ from solid Mg chunks is fairly complicated

My bet is that the experiment in question is about measuring the yield of the hydrogen, if so selection of the limiting reagent is quite obvious.