Is there a 'standard size' for volcanic eruptions in terms of gas output?
There is no "standard volcano" or "standard volcanic eruption". However, to perform the calculation you want to do, you could use the "typical" of "average" volcanic eruption. 

The characteristics of the "average" volcanic eruption can be found by calculating the average parameters of many volcanic eruptions. Such averages are not often reported, because the variability of volcanic eruption sizes and compositions is so large that averaging them doesn't makes much sense. But it does for your very particular application. So let's try to estimate how the "typical" volcanic eruption looks like:

Let's first figura out the typical volume of ejecta produced by a volcanic eruption: this is usually measured by the VEI (Volcanic Explosivity Index), and the formula to turn VEI into volume is

$\text{Ejecta volume}=10^{-4+\text{VEI}} \, \text{km}^3$ 

If we download NOAA's Significant Volcanic Eruption Database, it contains 646 recorded eruptions from 1750 to the present, and 498 of them have reported VEI values. Computing the volume ejected by each eruption (using the above formula) and taking the average we get that the "typical" eruption produces 3.2 km$^3$ of ejecta.

Using an average density of volcanic ejecta of 2 g/cm$^3$, this average volume corresponds to $6.4 \times 10^{12}$ g or 6,400,000 tons.

In typical magma, about 0.5% of its mass corresponds to gases, and for our "typical" mass that would corresponds to $3.2 \times 10^{10}$ g or 32,000 tons of gases in a "typical" volcanic eruption.

Now, about 7% of the gases corresponds to SO$_2$. That means that 0.035% of the mass of the ejecta is SO$_2$, and for our "typical" mass that would corresponds to 
$2.24 \times 10^9$ g or 2,240 tons of SO$_2$ in a "typical" volcanic eruption.

As you will see if you follow my references, I've pulled the above figures from multiple sources with varying degrees of reliability. Therefore, if you want a more precise estimate you can search for more accurate and updated values and repeat the exercise.