Condoms are measured in "nominal width", which is the size you get when you lay the condom over the edge of a rule.
It is therefore HALF of the circunference.
Wrap a piece of paper around the erect penis, near the body - mark where it overlaps itself.
Measure the length of the paper to the mark.
Half it.
Take off a couple of mm (to ensure the condom is not too baggy)
Get this size.
E.g. You measure 110mm around the penis.
Half of this is 55mm.
Take off a couple = 53mm - you need STANDARD condoms.
You measure 130mm around.
Half of this is 65mm.
Take off a bit, to get 60mm,
You need EXTRA LARGE condoms.
You measure 100mm around.
Half it to get 50mm.
Take off a bit - you get 48mm - you need CLOSE FITTING condoms.
You measure 60mm around,
Half it to get 30mm.
Take off a bit - you get 25mm.
You are a mouse - you should not be using condoms, go and find some cheese.
---------- Post added at 10:39 PM ---------- Previous post was at 10:30 PM ----------
Also - regarding spermicide. Don't.
Seriously - it makes NO difference to conception rates when condoms are used correctly, but slightly increases the chance of STI transmission.
Non-hormonal contraception - Questions and Answers | Bupa UK
http://archives.who.int/eml/expcom/expcom13/rhr.doc - which includes the following: "there is no evidence that condoms lubricated with N-9 are any more effective in preventing pregnancy than condoms lubricated with silicone. There is little evidence that this spermicide actually increases the contraceptive effectiveness of diaphragm use. The WHO UMC database stored 103 reports on unintended pregnancy in suspected association with N-9, which could be explained by the fact that N-9 is not an effective contraceptive."