One particular chapter in the MR4 manual covers sexing larvae using imaginal disks. Search 'sexing larvae pupae MR4'. With some species, females are significantly smaller larger or have a different color which is evident in the L4 particularly. Sexing stages younger than L4 will be tough. Best wishes.

http://www.mr4.org/Publications/MethodsinAnophelesResearch.aspx

Well, male larvae are slightly smaller in size than female larvae. So are the pupae. But it is not 100% correct.

There is no reliable method of sexing mosquito larvae short of cytological examination for the sex chromosomes. Size may be used if the larvae are reared under identical conditions, but size alone is not very reliable. Similarly male mosquito pupae are noticeably smaller than female pupae and this was exploited by Fay and Morlan (1959) who constructed a machine for high throughput separation of the sexes based on size. However, the pupae sexed by using this machine need to be further purified by removing the few small female;  even with pupae reared under identical conditions in lab size is approximately 95% - 99% accurate.  The most reliable character of sexing pupae is the genital pouch (the structure on the tenth abdominal segment). In male pupae the genital pouch is noticeably larger than that in  the female pupae.