Before I say to much there are several sites that have several different thoughts on the sizes and colors . There are several places that you can visit to get the information from. Please remember that these are all up for interpretation.
Can any of the things we call orbs be interpreted without a doubt? No, there is not certainty with the paranormal.
As for sizes there are also several different things that can count . Examples ; a large orb can be something that has an echo of the energies of a aura of one person or may even contain may souls. There is not certainty.
Red: Stress , restlessness , Energetic
Dark red : Anger and Pain
Orange: Healing, Energy , Health
Peach : Ability to comfort
Gold : Free-Flowing , Energy ,Tolerance
Yellow; Caution ,Hesitation
Green ; Healing , Fertility
Light Blue: Tranquility
Dark Blue: Shyness , Survivor
Lavender: At peace with God
Purple : Stored information
Violet : Seeking spiritual guidance
White: High Frequency protection
Silver : Telekinetic power
Brown : Connection to the earth