An average adult breathes about 0.5 L of air per breath at rest. Normal air contains ~ 0.03 % of CO2 which gets enriched to 4 % of exhaled air. The difference, 3.97 % is what you exhaled. One mole of air at room temperature have a volume of ~22.4 L (use ideal gas equation p*V=n*R*T, p=pressure [bar], V=volume, n=amount in mole, R=ideal gas constant, T=temperature [Kelvin]). 1 mole CO2 is equivalent to 44 g.
So, 0.5 L*(1 mole /22.4 L)*(44 g / mole) = 0.982 g

One breath therefore contains ~ 1 g of exhaled CO2

Note: Some of the gases are not ideal (mostly CO2), the volume of one breath depends on the person (sex, age, fitness, size) and the activity. It can be up to 2 L at least. But it is just an approximation anyways.

Btw - 1 kWh of electricity cost about 600 g of CO2. An average light bulb of 60 W therefore gives rise to the production of 10 g CO2 / s during use (not counting its production or disposal).
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If one breath is 4% CO2 and 1 breath is 0.5 liter, Mass of CO2 = 0.04*0.5 L* (1/22.4) mol/liter* 44 g/mol * (273 K/310 K) = 0.035 g/breath, not 1 g/breath. Body temp is 37 oC = 310 K, but that's a minor point.
If 1kWh results in 600 g of CO2, 60 Watts leads to 60W*600 g/kWH *(1/3600) h/sec *(1/1000) kw/W= .01 grams/second of CO2 not 10 g/sec. Otherwise people may feel guilty about breathing.

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some serious problems with the decimal place in the original answer. atmospheric content co2 is roughly 350 PPM or .00035, less than the original answer by several orders of magnitude. maybe that's why all the histeria about global warming, smoking in the boys room during middle school math....