It could be hydrocele but the only way to confirm is either by an Ultrasound of the Scrotum or by getting it examined by a Surgeon. The fact that the testicles decrease in size during cold is a normal response due to a muscle contraction called Cremasteric reflex. If you have a suspicion, then I suggest you get an Ultrasound done and then your worries can be set aside and if it shows hydrocele, then you can decide how you want to get treated - by Surgery or you can also observe it till it causes severe problems. Take care and let me know. Dr Rishi.