1) If it is from outside your country you will almost definatley have to sign for it.
2) If it is standard 1st, 2nd class etc you will not have to sign for it.
3) If it is using a method such as Recorded Delivery, Special Delivery, Express Post etc you will definatley have to sign for it.
4) Just to confuse you sometimes when the package is letterbox/postbox size Royal Mail will just post it if your not in as oppose to making you sign/collect it later when using the methods in 3) but if international will always keep ahold of the package untill you can provide some ID.
Not big at all i've had much bigger order's posted through, you should be fine although it depends on postman to postman whether they bung it through or not, some just want to get their rounds done with less paperwork and will shove it through others will stick to the book and post a card through so you'll have to go to a local depot or the like with some ID to collect, which I suppose you would want to avoid if possible.
"Depends on size.. my beans came in 4 days..right to mail box...no sig."
"my order for 4 Kali Mist seeds ... I had to sign for the package."
Might be different rules for different countries. The Yanks are fairly slack and us Canucks are fanatical about secure borders ... Safer to order in the US no doubt.