The size screw needed will depend on the style sender your engine uses as there are two sender designs which each one looks to use different size screws from new.
Typically new screws come with the sender kits with instructions of how to mount the sender...The instructions usually will also include the related part numbers of the kits accessories and sometimes the actual size of the bolts/screws and I just happen to have found them for you.
Not positive but the size I believe you may need would be Mercury part number 10-41069 which comes up as screw size (# 10-24 x .750). Here is the sender design that uses this size screw plus it's where I found the screw part number to help find the correct size...,
http://www.ebay.com/itm/Mercury-New-...dWMqpi&vxp=mtr
The other sender design uses Mercury screw part number 10-855341 which comes up as screw size (# 6-32 x .50) seen here,
http://www.ebay.com/itm/NEW-MERCURY-...ZSMg0Y&vxp=mtr
Plus here are some random close up online photos of how to mount the sender/wire if needed/interested,
http://s42.photobucket.com/user/Fish...nder7.jpg.html
