I found on Amazon:
As you can see, there are slight shape and size variations between the individual sponges; that's perfectly normal.
The two most frequent questions I'm asked about sponge use are:
1) How does it feel to use a sea sponge tampon?
Obviously, each person is different, but, as a general rule, if you insert it correctly--same motion used for inserting a tampon; push sponge all the way in, high up close to the cervix--you shouldn't be able to feel it.
Some women report a foreign body sensation after insertion (of either a tampon or a sea sponge). The most likely problem: the sponge sits too low in the vaginal canal.
To correct this, keep in mind that you have to insert the sponge along the vaginal axis. The axis is neither parallel nor perpendicular to the floor. This means you don't push the sponge straight in or straight up, in one motion. Rather, you use a two-step motion: start the insertion with fingers about parallel to the floor, continue and end the insertion with fingers in an upward direction. Something like this [please disregard the quality of my "art"; a bit crude but it does the job]:
2) Can I have sex with the sea sponge tampon in place?
Sure you can, the question is should you? As long as you're aware that the sponge tampon is not a birth control method and will not protect you against pregnancy (or STIs for that matter), the decision is up to you. Again, as a general rule, your partner will most likely not experience any discomfort from the sponge.
As always, keep in mind that this is just a brief blog post on using a sea sponge as a menstrual tampon. For a full discussion of the pros and cons talk to your own Ob/Gyn.