I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
For best results with a Mandevilla place it in full sun and keep the soil moist. The large pot is fine, but you will need to be extra careful that you are not keeping the soil too wet as saturated conditions excludes oxygen from the roots leading to root death and problems with root rot diseases. Fertilize the plant monthly with a complete soluble plant food such as Miracle Gro at the label rate.
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