There is not a guide per-say, but I'll give you my two cents.

As far as that toy, it should be fine providing you keep a good hold. Generally you want a base that is bigger than the insertable part, as that toy has. Looking at the toy though, that seam could cause some irritation or damage to the delicate tissues of the anus and rectum. Assuming you have use it vaginally, as long as you could not feel it there, you should be fine. I recommend a condom as well, that seem can and will harbor nasties and the vagina will not like them.

As far as other toys. The rectum is about (plus or minus) 7" deep, add at least three inches for a handle (more is better) so something in the 10-11" range is relatively safe. I would not consider anything shorter unless you are sure you can keep a good hold of it or attach a string or cord.

Harder materials do not conform to the rectum so they will be harder to get in all the way. That's not to say they won't, I have got my Njoy Eleven in all but two inches. Softer materials can bend around the various curves of the rectum and even enter the Sigmoid colon, so longer is better here.

What is boils down to is this, is it worth the risk of going to the ER? Are you sure you can keep a good hold of the toy? More often than not it is better to just buy a anal specific toy.

NO! Not an anal toy... so don't insert it! lol. I don't always say this with every toy, but for one... this toy has seams, for two, I don't think it was meant to be inserted, and third of all I owned this toy and it is a shitty plastic toy with small pieces of plastic that can hurt you if they weren't melted right.

Honestly, Ive always been a big fan of if it looks like it could be a problem assume that its going to be just to be safe.