You can politely tell the shadchan that you are looking for a slim healthy girl. Period. There is absolutely NOTHING wrong in letting the shadchan know that you are not attracted to girls on the heavier side. So just as long as you describe and emphasis YOUR preference in a prospective match, without actually directly saying anything negative about this girl in particular.
Lots of success and hatzlachah in finding your bashert soon!
mirkop6 – There is absolutely nothing wrong or immature about someone doing their homework and finding information about a shidduch. No, he DOESN’T have to ‘just go on a date to see for yourself’. Dates cost time, effort and money. It’s just as important to find out about a shidduch before as the actual date itself.