Organic reaction of dry ice and Grignard reagent
You are correct. Grignard reagents $\ce{RMgX}$ (where $\ce{R}$ stands for some hydrocarbon group and $\ce{X}$ is a halogen, usually $\ce{Cl}$, $\ce{Br}$, or $\ce{I}$) react with carbon dioxide to produce carboxylic acids after acidic workup:

$$\ce{RMgX -&gt;[1)\ \ce{ CO2}][2)\ \ce{ H3O+}] RCO2H}$$

$$\ce{RMgX + CO2 -&gt; RCO2- MgX+}$$
$$\ce{RCO2- + H3O+ -&gt; RCO2H + H2O}$$


  In general your reasoning when approaching this problem is a good one. If you are reacting two 1-carbon reagents, then you probably have a 2-carbon product.