Google trends is the best you can get. I'm not sure how you want to find the market size of anything, if no one is even aware of the product/product category. Particularly if you are looking for buyers. The market is still in the early phases of its life cycle.
Your best bet is to look at display advertising of the demographics who might respond to interruption marketing. In that case, these aren't people who are likely to buy immediately. You can make them aware of the product based on your followup strategy. Roger Wu's suggested approach is probably the closest you'll get.
What this comes down to is whether or not you have the budget, endurance and risk appetite to be a pioneer. And also you have to pick the right trend to jump on also. Think VHS vs BetaMax--in the early days both looked the same. The tech was comparable. But VHS became more popular so BetaMax faded out in relevance.
So to answer your original question, if there are few google searches for relevant terms, the current market size (of buyers) is small. The question is what market will be big in 5 years, which no online tool will tell you for sure. You're better off really understanding your target customers and choosing your target niche well, to minimize your onboarding cost and plan how you want to sequence the acquisition different adjascent niches...