On some toys there is a "view in real size" option that is very helpful! You put a credit card (or dollar bill, or ruler) up to your screen and adjust it to be the exact size as necessary, and the toy shown on screen will be the same size as it is in real life. I also have a measuring tape that I use if I'm not sure of how big something will be.

I judge the size from the specs while choosing. I even pull out my tape measure.

I don't dabble in strokers much- so can't be of much help.

For insertable toys where size can make it or break it- I use a tape measure in conjunction with my toy collection. I keep the specs on my plugs and dildos and assess what I am seeking in length/girth for future toy purchases.

The "view in real size" feature can be helpful but personally I have a very difficult time conceptualizing size from online images. Since I am fortunate enough to have a pretty large toy collection, I check the specs of new toys and compare them to the specs of my favorite toys. For instance, I know that my favorite insertable vibrators are usually at least 1.5in diameter, and some of my favorite favorites are 2in. Anything in the 1 or 1.25in spectrum is unlikely to be a favorite of mine. Length is a bit easier to imagine than girth; I will compare it to favorite toys but a tape measure or ruler also works pretty well. I recommend holding the ruler up against your hand to help imagine it.
Oftentimes when buying toys I have something specific in mind - e.g. "I like this vibrator that I already own but I want something with a little more length/girth," etc. Figuring out what you like or dislike about your current toys' measurements helps with picking new ones.

I used to go to the specs page, take out a tape measure (one you'd get in the crafts or sewing department that is foldable), and hold up the diameter or length measurements. However, if I was still unsure, I'd write down the measurements or mark them on my foldable tape measure, go to my nearest sex store, and find a toy that was closest in measurements. That gave me a visual of what to expect. Especially for plugs! I remember ordering a plug online and it was massive! I still can't use that one fully yet, but it makes a nice stretch for the next size down though.

A tidbit for the stroker question. Measure yourself before you buy. You don't want things too tight and since you'll get harder before orgasm, you'll want some room for growth. You can always go to your nearest sex store and ask an educated sales person about noise. Since toys cannot be returned, their policy should be to open the box, put batteries in it, and show you the noise level and that it works. Some can be pricey for quality. My husband just bought a pump for $100+ and when he brought it home to use it, he said it was great, BUT, when he popped it off his erect self, it made a sound like popping the top off a bottle and he was like "ouch!" I don't think he's used it since, but it may be just because he was ready to go and was in a hurry to take the pump off.